From MozillaZine Knowledge Base
This article deals with Firefox closing unexpectedly, or closing after an error dialog (e.g., the Mozilla Crash Reporter) appears.
If Firefox is crashing, first make sure you are using the latest public release version of Firefox, which will have the most recent fixes. You can download and install the latest version of Firefox from www.mozilla.org or you can use the Software Update feature from within Firefox. You may also wish to review the Firefox release notes for your Firefox version, to see if the issue and solution is listed under "Known Issues".
Crash logs may help you to locate the problem. See Crash reports, below.
Problematic extensions may cause crashes. Start in Firefox Safe Mode and click "Start in Safe Mode" ("Continue in Safe Mode" in Firefox 14 and below). This will help determine if an extension is causing the problem, since Safe Mode disables all extensions.
If Firefox no longer crashes when you run in Firefox Safe Mode, go to the Add-ons Manager Extensions list, click the Disable button for every extension, and restart Firefox normally. If the crashes no longer occur when you run Firefox in normal mode with all extensions disabled, one of your extensions was causing Firefox to crash. Try re-enabling them one at a time, to find the problem extension.
If disabling all extensions and restarting Firefox in normal mode doesn't resolve the crash problem:
Other installed software
Other installed software that is running on your computer may be causing Firefox to crash. In addition to the products listed below, Firefox crashes can be caused by Internet content-filtering or parental control software such as SafeEyes, McAfee Family Protection and similar products. If disabling the software stops the crashes, contact the product's support site see if there are any program updates or settings you can change to work around the problem (or uninstall the program, if you don't need it).  
BitDefender security software can cause crashes, particularly in Firefox 14 (Firefox 15, in Beta at time of writing, reportedly crashes much less frequently). A recent BitDefender automatic update should have fixed the problem so, if you use this software, make sure that the relevant BdProvider.dll file is version 188.8.131.526 (or above). The system should be restarted after the BidDefender update to insure that the program is using the updated files.   
BitGuard software by MediaTechSoft Inc. can cause Firefox to crash frequently or Firefox may crash at startup (bug 925459). BitGuard is bundled with toolbars or other software; if installed, you will see BitGuard.exe running in the Windows Task Manager. To resolve, remove this software via Windows Control Panel and check for malware.     
The SiteAdvisor component included with McAfee's Internet security software can cause Firefox to crash at startup, among other problems. (See also this blog post on a memory leak issue with the SiteAdvisor add-on.) Even though McAfee SiteAdvisor is installed as a Firefox extension, if Firefox crashes at startup you may not be able to remove or disable it from within Firefox Safe Mode. To resolve the problem, you can remove SiteAdvisor using the McAfee uninstaller (see McAfee's article, How to uninstall SiteAdvisor for details). Note: Some versions of McAfee SiteAdvisor are already on the Add-ons Blocklist.  
NVIDIA Network Access Manager
The NVIDIA Network Access Manager, an optional application for network security included with some NVIDIA products, can cause Firefox to crash unexpectedly. An easy workaround is to uninstall or disable this program and use another firewall, such as the built-in firewall included with Windows 7.   
The RoboForm Password Manager application for Windows can cause Firefox to crash; for example, older versions would crash at (or close to) startup (RoboForm 7.6.1 and below, now blocklisted in Firefox 8 and above) and recent reports show that RoboForm 7.7.3 causes random crashes. If you need this application, check for an updated version on the RoboForm for Firefox page or from http://www.roboform.com/download.   
Spector Pro for Windows
Spector Pro for Windows computer monitoring software can cause Firefox to crash at startup or when opening a new tab. If you use this product, try updating to the latest version or contact SpectorSoft Support.   
PC Tools' ThreatFire anti-malware software can cause Firefox to crash at startup. Try updating ThreatFire to a newer version or uninstall it.   Note: If you have both ThreatFire and Avast software installed, see this article.
ZoneAlarm ForceField (either standalone or included with ZoneAlarm Extreme Security) may cause Firefox to crash on certain websites or crash at startup. Try clearing the ZoneAlarm virtual cache (depending on ZA version, go to "Browser Security -> Settings -> Advanced" or "Internet -> Web Security -> Settings") then click "Clear Virtual Data" and restart Firefox. If you still have problems, turn off Forcefield (in ZA Extreme Security, go to "Internet -> Web Security -> Settings -> Advanced Settings", and turn off "Enable Virtualization").   Note: If Firefox fails to start after an update, see Firefox does not start after updating with ZoneAlarm ForceField enabled.
Crash when starting Firefox
Crashes that occur when starting Firefox are difficult to troubleshoot, especially if you also cannot start in Firefox Safe Mode. Make sure you allow the Mozilla Crash Reporter to submit a crash report so that you can view it later to get any useful information about the crash, including related bug reports (see viewing crash reports).
Note: If Firefox doesn't start up but you aren't sure whether it crashed or not and you didn't see the Mozilla Crash Reporter, see also Browser will not start up.
If Firefox does not crash when you restart in Firefox Safe Mode (at the "Firefox Safe Mode" window, click "Continue in Safe Mode" or, in Firefox 15 or above, "Start in Safe Mode"):
If Firefox also crashes when you attempt to restart in Firefox Safe Mode:
Crash when closing a tab or exiting Firefox
This can be caused by the Norton Toolbar extension included with Norton software such as Norton 360, Norton Internet Security and Norton Identity Safe. Your Norton product may have an update that resolves the issue (see this Norton community thread for current Firefox 18 compatibility updates). Otherwise, try disabling Norton Safe Search in the Norton (Toolbar) menu as a workaround. If Firefox still crashes, disable the Norton Toolbar extension in the Firefox Add-ons Manager (which you may need to do from Firefox Safe Mode) then exit Firefox completely and reopen it.    
Crash when downloading
Crash when choosing a file or folder location - Windows
If Firefox crashes when attempting to select a file or folder location, such as when backing up bookmarks, browsing to a file to upload it, or choosing a location to save a file, this can occur if you have selected "Automatically expand to current folder" in your Windows Folder Options (applies to Windows 7). To resolve the problem, go to the Windows Start menu, click on "Computer" and in the window that opens, click on "Organize -> Folder and search options" and uncheck "Automatically expand to current folder" in the Folder Options "General" tab.  
Damaged plist file - Mac OS X
On Mac OS X, sometimes a recurring crash pattern is caused by a preference file being damaged. Quit Firefox, then go to your user Library -> Preferences, find the file "org.mozilla.firefox.plist" and move it to your desktop (don't trash it; just leave it on the desktop). When you restart Firefox, a new .plist file will be generated. If the new preferences file is working well, you can trash the .plist file on the desktop. More information on corrupt .plist files can be found here. 
Certain video devices or display drivers can cause crashes. Make sure your drivers are up to date. Starting in Firefox 4, details about your video device and driver can be found in the Graphics section of the Troubleshooting Information (about:support) page. Note: In Firefox 4 and above, turning off the option to use hardware acceleration ("Tools -> Options -> Advanced / General -> Use hardware acceleration when available") may prevent video-related crashes.   
Note: Most plugins run in a separate "plugin-container" process by default so that instead of crashing the Firefox browser, only the plugin process will crash. See Plugin-container and out-of-process plugins for more information.
Plugins are programs that work within Firefox and allow you to experience multimedia content such as video, animation or sound. Plugins can crash within their own code or can cause a crash within Firefox code. Try updating your plugins to the latest versions, which you can do through Mozilla's plugin check. You can also temporarily disable your installed plugins to see if the crashes stop, either by disabling the plugin via "Tools -> Add-ons -> Plugins" (Firefox 3 and above) or by renaming or removing the associated plugin file. See Issues related to plugins for details on troubleshooting plugin issues.
Windows compatibility mode
On Windows XP, sometimes Firefox must be run in Windows 2000 compatibility mode. Right click on your Firefox desktop icon and click on Compatibility > change the setting to Windows 2000 > click Apply.
Still experiencing problems?
Mozilla Crash Reporter crash reports may help you locate the problem (see the linked articles for detailed information). For help analyzing the crash information, post on the Firefox Support forum with your Report ID and what steps cause Firefox to crash. Those who read your post may be able to look up your crash and find an existing bug report or file a new one for you. Also, if the crash you experience is common, it may be marked as a "topcrasher", making it more likely to be fixed in the next Firefox release.
Your operating system's system log may also have a crash report. The information will probably include the name of the module that caused the crash. On recent Windows versions, go to Start > Settings > Control Panel > Administrative Tools > Event Viewer. In the application tab, right-click on the event and select Properties. When you report the event, you should copy the information from the log.