Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes).
See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

If it does work in Safe-mode then disable all your extensions and then try to find which is causing it by enabling one at a time until the problem reappears.
You can use "Disable all add-ons" on the Safe mode start window to disable all extensions.
You have to close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

You also have out-of-date plugins with known security issues.

• Adobe PDF Plug-In For Firefox and Netscape "9.3.4"
• Shockwave Flash 10.1 r82
• Next Generation Java Plug-in 1.6.0_17 for Mozilla browsers

1. Check your plugin versions: http://www.mozilla.com/en-US/plugincheck/
   • Note: plugin check page does not have information on all plugin versions
2. Update Adobe Reader (PDF plugin):
   • From within your existing Adobe Reader (if you have it already installed):
     • Open the Adobe Reader program from your Programs list
     • Click Help > Check for Updates
     • Follow the prompts for updating
     • If this method works for you, skip the "Download complete installer" section below and proceed to "After the installation" below
   • Download complete installer (if you do NOT have Adobe Reader installed):
     • Use Firefox to download and SAVE the installer to your hard drive from the link in the article below
     • On the Adobe page, un-check any extra items (i.e. Free McAfee® Security Scan Plus) then click Download button
     • If you see message just under tabs, DO NOT click the Allow button, instead click below that where it says "click here to download".
     • Click "Save to File"; save to your Desktop (so you can find it)
     • After download completes, close Firefox
     • Click the installer you just downloaded and allow the install to continue
       • Note: Vista and Win7 users may need to right-click the installer and choose "Run as Administrator"
     • Download link and more information: https://support.mozilla.com/en-US/kb/Using+the+Adobe+Reader+plugin+with+Firefox#Installing_and_updating_Adobe_Reader
   • After the installation, start Firefox and check your version again.
3. Update Shockwave Flash
   • Use Firefox to Download and SAVE to your hard drive from one of the links below; the links take you directly to the download and you avoid the "getplus" download manager and "extras" (i.e., toolbars, virus scan links, etc.) on the main Adobe page.
   • SAVE to your Desktop so you can find it
   • After download completes, close Firefox
   • Click on the file you just downloaded and install
     • Note: Vista and Win7 users may need to right-click the installer downloaded and choose "Run as Administrator"
     • Note: Most browsers other than IE will also get updated with this one download
     • Note: To update IE, same procedure but use IE to go: http://fpdownload.macromedia.com/get/flashplayer/current/install_flash_player_ax.exe
   • After installation, restart Firefox and check your version again.
   • Download links and other information:
     • https://support.mozilla.com/en-US/kb/Managing+the+Flash+plugin#Updating_Flash
     • or you can download, save, then run the manual installers for IE, then for Firefox (and all other browsers) from here: http://kb2.adobe.com/cps/191/tn_19166.html#main_ManualInstaller
   • Also see: http://support.mozilla.com/en-US/kb/Installing+the+Flash+plugin
   • Also see (if needed): http://kb2.adobe.com/cps/191/tn_19166.html#main_Uninstall
4. Update Java:
   • Download and update instructions: https://support.mozilla.com/en-US/kb/Using+the+Java+plugin+with+Firefox
   • Removing old versions (if needed): http://www.java.com/en/download/faq/remove_olderversions.xml
   • After the installation, start Firefox and check your version again.

Thanks all for your replies. I initially followed Cor-el's advice and found the culprit to be FinalVideoDownloader plugin for Mozilla Firefox 1.0.1

If I remember correctly, that is used to download You Tube videos and I have found it very useful. I have disabled it for now, but I may try to find an updated version and hope that it will not mess up Firefox.

I'm having this problem too (mac) and the google toolbar just disappeared. I got it back (following the tips here) but... when I click on "add/edit" profile under the autofill icon in google toolbar, I get nothing. it lets me click but then nada... no profile pulls up, I can't edit autofill or use it to autofill anything. so frustrating. I didn't realize how much I use autofill. Should i uninstall 3.10 and 3.11 (and umm how would I do that?) that's when the problems started and I haven't seen any positive changes in ff 3.10/3.11 that are worth the inconvenience of losing google toolbar.

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes).

See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems