From MozillaZine Knowledge Base

Hanging (freezing, not responding) is when a program is running but stops responding to user input. A hang/freeze is different from a crash, which ends the program.

If Firefox closes unexpectedly, or if an error dialog appears and Firefox closes, see Firefox crashes.

Hang loading first Firefox window

Hyper-Threading

Hyper-Threading is a feature on some Intel processors. In some cases, motherboard chipsets supporting Hyper-Threading can cause Firefox to hang at startup. If you’re experiencing hangs, you may want to check your motherboard manufacturer’s website for updates to the BIOS.

Alternatively, you can disable Hyper-Threading support for Firefox in Windows 2000 and XP. Right click the Firefox shortcut → Properties → Compatibility → Check "Run this program in compatibility mode" → Select "Windows 98/Windows ME" → OK.

Hang downloading files

As the download history grows, Firefox may download files more slowly and even appear to hang during downloading. In the Firefox menu, click "Tools -> Downloads -> Clean Up" to clear the Download History. If this doesn't help or if it also causes a hang, exit Firefox completely and delete the downloads.rdf file in the Firefox profile folder [1]. Selecting a different download folder in your Firefox Options may also help. See this article for more information.

Hang saving images

See above.

Hang using page history

If you set Firefox to remember the visited pages for more than the default 9 days, operations involving history can hang Firefox for short periods of time. Setting the history to 9 days or less and clearing the history (Tools -> Options -> Privacy -> History: "Clear Browsing History Now") will usually resolve the problem [2]. Note that the "Clear Browsing History Now" feature is no longer available in Firefox 2 but can be cleared as part of the "Clear Private Data" function, if "Browsing History" is selected in "Tools -> Options -> Privacy / Private Data -> Settings".

Hang after prolonged or extensive use

For some users, Firefox consumes excessive amounts of memory and may slow down or hang in situations that include being open for long periods of time, after downloading or saving images or other files, or after extensive use with multiple tabs open. See the article, Reducing memory usage - Firefox. Additional suggestions are given here.

Hang loading websites

Hang loading sites with JavaScript

Firefox may become very slow to respond because of badly coded JavaScript causing infinite loops on certain websites [3]. This problem can usually be cleared by closing the window or tab for that website. You can also temporarily disable JavaScript to view the problem site, but the website may not work without JavaScript. Read Firefox CPU usage for more information and also see the related article, Unresponsive Script Warning.

If the web site hangs Firefox (you can't even exit Firefox) try running in Safe Mode and see if you can get an Unresponsive Script Warning. Watch the status bar and see if this frequently occurs while it mentions a specific URL. If so, try blocking that URL using a content-blocker such as Adblock Plus. The problem URL may be fetching an advertisement or side panel that you can do without.

If none of this helps, try using a different browser (such as IE or Opera) or try accessing the site at another time. This might help you determine if the problem lies with Firefox, a poorly designed website, or content that changes over time, such as ads. [4]

Hang loading sites with Java, Flash, or PDF files

If Firefox hangs attempting to load a site with Java or Flash, or an Adobe PDF file, it's likely a problem with the associated plugin. If Firefox hangs after waking the computer from sleep mode, it may be a known problem with the Flash plugin.

Hang loading websites when using an automatic proxy configuration file

If you're using an auto-config proxy (PAC), Firefox will hang when you attempt to load sites that don't exist and when opening sites on a domain you haven't opened recently. The amount of time it hangs will depend on how many look-ups the PAC does. There is no current solution, however this is a recognised bug, and as such is being tracked here: Bug 235853 - PAC: isResolvable() causes the UI to hang during resolution.

Other causes of hanging

Extensions

Extensions are a common cause of Firefox problems, including hangs, slowdowns, or periodic freezes. Check the list of problematic extensions to see if it is a known issue and go through the standard diagnostic steps related to extension issues to see if an extension is causing Firefox to hang.

Multiple sessionstore.js files

If Firefox is slow to respond or appears to hang, open the Firefox profile folder, check for multiple sessionstore.js files and delete all copies found. [5] See Multiple profile files created for details.

Spybot Search and Destroy

Spybot S&D includes an "Immunize" feature that adds a large number of entries into the Firefox hostperm.1 file. On some systems, this can cause Firefox to respond very slowly; for example, extended pauses when loading web pages or switching tabs. [6] [7] To undo the Firefox immunization, go into Spybot S&D -> Immunize. If you right-click in the Immunization window on the right, you can select "deselect all". Checkmark the Immunizations for Firefox you would like to remove, then click Undo. See this Spybot forum topic for more information.

Norton Anti-virus

Older versions of Norton Anti-virus may cause Firefox 2 to hang when NAV real-time protection is turned on. Updating to the latest version of NAV may solve the problem. [8]

Outpost Pro Firewall

Outpost Pro Firewall, versions 2.7 and 3.0, can cause Firefox to stop loading Web sites. The "firefox.exe" process may also be left running after closing Firefox, and shutting down the computer may be blocked by an error message that the XPCOM:EventReceiver is still waiting for data.[9],[10] This may be due to blocking the loopback (localhost) connection.[11]

Adobe Type Manager/Font issues

Having installed a huge number of fonts can also cause this problem. Using Adobe Type Manager can lead to a huge number of installed fonts. Uninstall all fonts - except the standard fonts - and Adobe Type Manager in this case.

Windows Compatibility mode

Running Firefox in Compatibility mode for an older version of Windows (e.g. Windows 95 Compatibility mode) can cause Firefox to run very slowly. Check the Compatibility tab in the Properties of the Firefox desktop shortcut and uncheck "Run this program in Compatibility mode" if it is checked. [12].

Other solutions

• Perform the standard diagnostic to resolve hanging problems not specifically mentioned in this article.
• Some older versions of video card drivers can cause hangs. Make sure your drivers are up to date.
• If none of the solutions on this page fixes your problem, you may want to file a bug report. This is a list of unsolved bugs in Firefox that cause hangs. Look through the list, and if your situation doesn't match any of the situations listed, file a new bug report. If you cannot give a set of steps that reproduces the problem, you may want to try to find a regression window to determine which change to Firefox caused the problem. On Windows, you can also use Process Explorer to determine which thread is using the most CPU.

See also

• Firefox CPU usage
• Error loading websites

External Links

• Crash Firefox to determine where Firefox is hanging
• Firefox hangs (Firefox Support)