I'm on Firefox 5.0 and just got a popup, translated from Swedish:
A security and stability update for Firefox has been downloaded and is ready to be installed.
Update: Firefox 6.0
Is Firefox 6.0 -- a major version higher than 5.0 -- really a security and stability update? I'd rather not update since I'm happy with 5.0 (I thought I was on 3.0 before I checked), but if it's a security update, it doesn't leave me much of a choice.