The Facebook Help Center has a troubleshooting page that lists known Facebook website problems and bugs. If you see your specific problem listed on that page, Facebook is aware of it and is working on a fix. If your problem isn't listed, follow the troubleshooting steps below. Clear cookies and cache

Clearing your stored cookies and the Firefox cache can solve several common problems with Facebook, such as Farmville issues.

   At the top of the Firefox window, click on the Firefox button, go over to the History menu and select Clear Recent History....On the menu bar, click on the Tools menu, and select Clear Recent History....At the top of the Firefox window, click on the Tools menu, and select Clear Recent History....
   In the Time Range to clear: drop-down, select Everything.
   Click the arrow next to Details to display the list of items that can be cleared.
   Select both Cookies and Cache.
   Click Clear Now. 

   At the top of the Firefox window, click on the Firefox button, go over to the History menu and select Clear Recent History....On the menu bar, click on the History menu, and select Clear Recent History....At the top of the Firefox window, click on the History menu, and select Clear Recent History....
   In the Time Range to clear: drop-down, select Everything.
   Click the arrow next to Details to display the list of items that can be cleared.
   Select both Cookies and Cache.
   Click Clear Now. 

   Click the menu button New Fx Menu , choose History and select Clear Recent History....
   In the Time Range to clear: drop-down, select Everything.
   Click the arrow next to Details to display the list of items that can be cleared.
   Select both Cookies and Cache.
   Click Clear Now. 

Check cookie settings

If you receive an error message that mentions cookies after clearing your cookies and cache:

   At the top of the Firefox window, click on the Firefox button and then select OptionsAt the top of the Firefox window, click on the Tools menu and then select OptionsOn the menu bar, click on the Firefox menu and select Preferences...At the top of the Firefox window, click on the Edit menu and select Preferences

   Click the menu button New Fx Menu and choose OptionsPreferences
   Select the Privacy panel.
   Set Firefox will: to Use custom settings for history.
   Make sure Accept cookies from sites is check marked.
   Make sure Accept third party cookies is check marked.Make sure Accept third party cookies is set to Always or From Visited.
   Click Exceptions….
   Make sure facebook.com is not listed.
       If it is listed, click on its entry, then click Remove Site. 

Check JavaScript settings

Facebook errors can be caused by problems with JavaScript. JavaScript is enabled by default in Firefox, but to check its status:

   At the top of the Firefox window, click on the Firefox button and then select OptionsAt the top of the Firefox window, click on the Tools menu and then select OptionsOn the menu bar, click on the Firefox menu and select Preferences...At the top of the Firefox window, click on the Edit menu and select Preferences

   Click the menu button New Fx Menu and choose OptionsPreferences
   Click to select the Content panel.
   Make sure that Enable JavaScript is checked.
   Click OK to close the Options windowClick Close to close the Preferences windowClose the Preferences window 

You should also check whether you have an extension (such as NoScript) or Internet security program (such as a firewall, antivirus, or anti-spyware program) that can block JavaScript. Check if JavaScript is being blocked

You should check whether you have an extension (such as NoScript) or Internet security program (such as a firewall, antivirus, or anti-spyware program) that can block JavaScript. Check extensions, themes, and plugins

If the steps above did not fix your problem with Facebook:

   Determine whether an extension or theme is causing your problem. See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems for instructions.
   Determine whether a plugin is causing your problem. See Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems for instructions. 

Check for malware

Certain types of malicious software can cause problems with connecting to Facebook. See Troubleshoot Firefox issues caused by malware for instructions on troubleshooting malware problems with Firefox. Changes to Facebook

Facebook is known to make changes to their system regularly. Check Facebook Developer Blog for changes that have recently been made.

Does it make a difference if you disable the e.dentifier2 Plug-in for Mozilla and enter the response code manually?