Firefox and Thunderbird tend to have profile corruption over time. Mozilla profiles can have huge, profound effects upon how the application acts.
You should try to make a new profile.In order to create a new profile, you use the Profile Manager. To start the Profile Manager in Windows, follow these steps:

1. Close Firefox completely (select File > Exit from the main menu of Firefox).
2. Select Start > Run... from the Windows Start menu (use the search box on Vista).
3. Enter firefox.exe -ProfileManager and press OK.

On Linux or Mac, start Firefox with the -ProfileManager switch, e.g. ./firefox -ProfileManager (this assumes that you're in the firefox directory).

You should now see the Profile Manager window, shown in the screenshot to the right.

From the Profile Manager you are also able to remove and rename profiles. Be very careful when deleting profiles; if you created the profile in an directory that already existed, the entire directory will be removed!

Click on the Create Profile... button to start the Create Profile Wizard. Click Next and enter the name of the profile, e.g. your name or something descriptive.

You can also choose where on the disk you want the profile to be stored, which is useful if you plan on exporting your settings to another computer or setup in the future. If you choose a custom location for the profile, always create a new/clean directory and select that directory.

Finally, click Finish to have Firefox create the new profile.
You should now be taken back to the Profile Manager and the newly created profile should be listed. Select it and click Start Firefox. That's it!

You are now running Firefox with the new profile, which means all settings are reset to default. If you want to switch back to your old profile, just start the Profile Manager again (instructions above) and select the old profile.

You can also have Firefox start a selected profile automatically, so you don't have to pick one each time the browser is launched. Do this by checking the Don't ask at startup option.

If that didn't help try the instructions in this article: http://kb.mozillazine.org/Images_or_animations_don't_load