It’s quite annoying that extensions like YSlow were perfectly working with Firefox 3b5 and are now broken with Firefox 3rc1. Yes, extensions are designed for specific Firefox versions, but they can often work without any change, but ignoring the version.
Here’s a quick and dirty way to let Firefox ignore versions.
Type about:config and add a new boolean called extensions.checkCompatibility and set it to false.
Restart Firefox. Done.