If opening the Downloads window in Firefox or downloading files are freezing the program, try deleting your download history. If this still causes a hang, try this fix from the MozillaZine Knowledge Base:
- Find your profile folder. In Windows XP it’s here: C:Documents and Settings<Windows login/user name>Application DataMozillaProfiles<Profile name>********.slt%APPDATA%MozillaProfiles<Profile name>********.slt
- Find and deete the file downloads.rdf.
Firefox should work fine now.