From MozillaZine Knowledge Base
Toolbar add-ons from third-party software providers can be installed in Firefox by visiting a website and clicking an "Install' link or, in some cases, by downloading and running a toolbar installer. Toolbars can also be added to Firefox when installing software that includes a toolbar as part of the installation process.
Note: Be careful when installing or updating other applications, which may install a toolbar by default unless you deselect the toolbar option. For example, the Java installer can include a preselected option for the Yahoo! Toolbar for Firefox/Mozilla  and the Avira Antivirus installer may add the "Avira SearchFree Toolbar plus WebGuard" (Ask.com toolbar) to Firefox.  See How Programs Trick You Into Installing Adware On Your Computer for more information and examples.
If you just want to hide an unwanted toolbar, you can open the Firefox "View -> Toolbars" menu and deselect the toolbar you wish to hide by clicking the checkbox to clear it (see Toolbar customization for more information).
Note: Starting in Firefox 4, the Menu bar is hidden by default on Windows 7/Vista. You can press the "Alt" key to temporarily show the Menu bar, then open the View -> Toolbars menu, or you can open the orange Firefox Button drop-down menu and go over to the Options sub-menu to de-select the toolbars you want to hide, as shown below.
To uninstall an unwanted third-party toolbar from Firefox, use one of the methods listed below.
In some cases, you may also need to reset preferences or Search Bar settings related to the toolbar after disabling or uninstalling it. In particular, toolbars that include an Internet search feature (such as the Ask.com Toolbar) can change the default search provider by modifying the Keyword.URL preference or by changing the default search engine.  
If the following instructions don't work, you should seek help from the toolbar provider.
Many third-party toolbars are installed as Firefox extensions, which can be uninstalled from the "Add-ons -> Extensions" list (accessible from the Firefox button or Tools menu), selecting the toolbar, clicking "Remove" button ("Uninstall" button in older Firefox versions) and then restarting Firefox. If the "Remove" option is not working, restart Firefox in Safe Mode and try again. If you still can't uninstall the toolbar, use the 'Disable" option as a workaround, or see below for other solutions. Note: If you can't see any of your installed extensions in the Add-ons Manager Extensions list, the associated files may be corrupt. To fix this, see Corrupt extension files.
Uninstalling via Windows Control Panel
You may be able to remove the toolbar from the Windows Control Panel list of installed programs;
If you don't see a separate entry for the toolbar in the list of installed programs you may need to uninstall the program or program component that included the toolbar.
Some toolbars can be uninstalled using the settings available within the toolbar itself. For example the Google Toolbar for Firefox can be uninstalled by clicking the "Settings" button in the Google toolbar or, in older versions, by clicking the "Google" logo. You can then choose "Help" and then "Uninstall."  Note: the Google Toolbar is no longer supported by Google for Firefox 5 and above (see Using Google Toolbar features without toolbars). Also, some versions of the Yahoo! Toolbar have a “Settings” button with an uninstall menu option. To see which version of the toolbar is installed, visit this Yahoo Help page. If you have a "Pencil" button, click "Uninstall" in the menu, enter any feedback, then click "Uninstall" and restart Firefox. 
If the toolbar was added to Firefox when you installed another application, see if there is an option to remove or disable the toolbar within the application's own settings.
Some toolbars must be removed in steps, some of which may include manual file deletion. For example, if you want to remove a toolbar extension from the Firefox Add-ons Manager instead of just disabling it, you may need to find and manually delete the installation folder or files.
First remove the Babylon Toolbar from the Windows Control Panel list of installed programs, if listed (see above). Next, remove it from the Firefox "Add-ons -> Extensions" list (see above) and restart Firefox, then remove Babylon (if found) from the Search Bar "Manage search engines" list. Note: If the Remove (uninstall) button for the Babylon Toolbar in the Firefox Add-ons -> Extensions list is greyed out, either use the "Disable" button as a workaround or else find and delete the Babylon Toolbar installation files from the installation directory\extensions folder (the Babylon Toolbar extension ID can be found in the "Help -> Troubleshooting Information" about:support page). For example, according to this recent Babylon Toolbar removal guide, on 64-bit Windows 7 you would manually delete the "firstname.lastname@example.org" folder located here: C:\Program Files (x86)\Mozilla Firefox\extensions\ email@example.com.
You may also need to reset your Firefox home page in the Options window General panel and you may need to search for "Babylon" entries in the about:config list and reset keyword.URL and other modified preferences. Alternately, use the Search Reset extension to reset your search preferences and Firefox home page to the default settings or reset all user preferences to Firefox defaults. If the Babylon Toolbar or its search and home page keeps coming back after removal, check for a user.js file in your profile folder. In some cases, Babylon settings may have been added via the user.js file so you'll need to either delete that file or remove the Babylon entries. See Resetting preferences for more information. Also, if you see a browsemngr.exe process running in Windows Task Manager or if Firefox crashes and the crash report mentions browsemngr.dll you will need to remove the "Browser Manager" software that is apparently reinstalling Babylon. You should also check for malware.        
Norton 360 and Norton Internet Security currently add an anti-phishing toolbar to Firefox. To remove the Norton toolbar in older versions (e.g., Norton 360 2.0) exit Firefox completely and remove (or rename) the file "coFFPlgn.dll" from the Firefox installation directory components folder (usually C:\Program Files\Mozilla Firefox\components).  If the file is regenerated when you restart the computer, remove it again and in its place, create an empty text file (e.g., with Notepad), rename it "coFFPlgn.dll" and change the properties to "read-only".   Note: In recent Norton toolbar versions (e.g., Norton Toolbar 4.5), "coFFPlgn.dll" is no longer added to the Mozilla Firefox components folder (related to bug 519357). Either disable the Norton Toolbar in Firefox from the "Tools -> Add-ons -> Extensions" window, or else try the workarounds given here and here.
Customized versions of Firefox
You may have installed a customized version of Firefox with an integrated toolbar. For example, Yahoo is offering a "Mozilla Firefox ® optimized for Yahoo!" here that includes Seamless integration of Yahoo! Search, Homepage, and Toolbar. According to this Mozilla blog post, Firefox has nearly 20 customized versions distributed by partners around the world and mentions MSN, AOL, Bing, Twitter, Yahoo! and Yandex versions. To resolve, uninstall Firefox, remove any leftover Mozilla Firefox program files and then do a clean reinstall by running a Firefox setup file downloaded from mozilla.org.