add-ons in firefox 3? this is what i had done already:
1. i have mcafee anti-virus
2. i've just installed firefox 3 then updated it to the latest version 3.0.1
3. tried installing my first add-on but error invalid file hash
4. cleared cache, disabled anti-virus, firewall off, deleted the 3 extensions files in my application data folder, third party cookies already checked... still error
6. manually downloaded the .jar or .xpi file then drag onto firefox 3... same error
7. checked to remove any existing add-ons/extensions before trying again... nothing to remove because its a fresh install
8. trying to think its just a problem with the mozilla database or broken mirror to relieve thyself... i still think its not
9. started banging my head on the keyboard afterwards... error
i'm still thinking that there's no problem with the mozilla database or a broken mirror because according to forums i've visited, some people are able to install them no problem.
i got it! i tried using a different manual install procedure and it works now. i still want the normal firefox install to work though...

File hashing is a way for computers to maintain consistency with files when they're transfered from computer to computer. That way you don't have a file named the same thing but of different sizes and containing different data. When you get a hash error, it's telling you it was expecting data (or not expecting data) that was there. If you're having trouble with addons, it may be a problem with the source (unless it's verified by mozilla, in which case you should contact their support group).