Backup your bookmarks in your firefox profile, delete your profile, start firefox again, quit firefox, put your bookmarks back in the new, fresh firefox profile. Starting with a fresh profile often fixes strange firefox problems in my experience.
Your profile will be located at:
"C:\Documents and Settings\(your username)\Application Data\Mozilla\Firefox\Profiles\(a bunch of random letters/numbers followed by .default" for windows xp, and
"C:\Users\(your username)\AppData\Roaming\Mozilla\Firefox\Profiles\(a bunch of random letters/numbers followed by .default" for windows vista .
Your profile is the "bunch of random letters/numbers .default" directory. Your bookmarks should be named "bookmarks.html" in that directory, if you want to back them up to keep them.
