Did you start Firefox in Safe mode ?

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

You can also try to delete localstore.rdf the Firefox Profile Folder

See http://kb.mozillazine.org/Corrupt_localstore.rdf

awesome thanks. deleting the localstore.rdf did te trick!

You're welcome

Sorry, but neither solution works in my environment (Windows Vista, Firefox 3.6.10). Since years I had similar problem with IE, since 2 days also in firefox.

Did you find the Firefox Profile Folder ?

Help > Troubleshooting Information > Profile Directory > Open Containing folder

Create a new profile as a test to check if your current profile is causing the problems.

See Basic Troubleshooting: Make a new profile

There may be extensions and plugins installed by default in a new profile, so check that in "Tools > Add-ons > Extensions & Plugins"

If that new profile works then you can transfer some files from the old profile to that new profile (be careful not to copy corrupted files)

See http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox