I can't log to certain sites (facebook, hotmail) anymore.
I've tried almost all of your tips. The problem is solved if I use Mozilla in the safe mode. However, none of the next suggested steps (disable themes, extensions etc...) worked. The first time this occurred, it was solved by chance when a website asked to download some new java"thing". Also, consider my operating system is windows 2000
URL of affected sites
Create a new profile as a test to check if your current profile is causing the problems See [[Basic Troubleshooting|#Make_a_new_profile|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