me too. This did seem to help:

Try clearing Firefox's cookies and cache

Do these steps and then reload the website you're having trouble with.

   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. 

If clearing Firefox's cookies and cache didn't work, it's probably a sign that there is a problem with the website itself. In that case you'll just have to wait for it to get fixed. With big sites like Twitter or Facebook this may only be a few minutes.

This is likely a problem with corrupted cookies from that website, so you should definitely clear the cookies from that website.

There are other things that need attention.

The System Details List shows that you have multiple Flash plugins.

1. Shockwave Flash 20.0 r0
2. Shockwave Flash 9.0 r45

You can find the installation path of all plugins on the about:plugins page.

• http://kb.mozillazine.org/Issues_related_to_plugins#Identifying_installed_plugins

You can check the Flash player installation folder for multiple Flash player plugins and remove older version(s) of the plugin (NPSWF32) and possibly (re)install the latest Flash player.

• (32 bit Windows) C:\Windows\System32\Macromed\Flash\
• (64 bit Windows) C:\Windows\SysWOW64\Macromed\Flash\

You can find the latest Flash player versions for Firefox on this page under "Still having problems".

• http://www.adobe.com/special/products/flashplayer/fp_distribution3.html
• http://helpx.adobe.com/flash-player/kb/installation-problems-flash-player-windows.html