Perform the suggestions mentioned in the following articles:

• Clear Cookies & Cache
• Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Check and tell if its working.

May not be related to your problem but some of your Firefox Plugins are out-dated

• Update All your Firefox Plugins -> https://www.mozilla.org/en-US/plugincheck/
• When Downloading Plugins Update setup files, Remove Checkmark from Downloading other Optional Softwares with your Plugins (e.g. Toolbars, McAfee, Google Chrome, etc.)

I had the same problem. Gmail and Yahoo mail were also displaying their basic HTML versions. So I searched the web a bit and came upon this article:

https://support.mozilla.org/en-US/kb/.../latest-firefox-issues#w_websites-say-i-need-to-update-firefox-but-im-on-the-latest-version

Basically it describes how to install the User Agent Switcher Add-on. I did this, switched to Firefox 9, and everything is back to normal.