Unofficially the RC2 was available to download for the past week, but yesterday it is released officially.
Upgrading From Firefox 3.0 RC 1 To RC2
So is it required to upgrade to Firefox 3 RC 2? Even though they are no new features in RC2 when compared with RC1, it is worth upgrading as many bugs identified in RC1 are fixed.
Upgrading From Firefox 2 to Firefox 3 RC 2
You have to think carefully before upgrading to Firefox 3.0 from Firefox 2. Installation of Firefox 3 overrides your previous version without loosing any bookmarks, preferences and other settings. But most of the Firefox 2.0 add-ons are not compatible with Firefox 3.0. Few my favourite add-ons that does not work in Firefox 3.0 are Google Toolbar, Fireuploader and Autocopy.