After installing Firefox 5, replacing my previous version (4.5), I could see the new features. However, having closed and reopened Firefox, it seems as though it has reverted to the previous version, even though 'Help - About' says I'm running on version 5.0.

I had to download 5.0 from the Mozilla website, as I tried to update via the update tool in 4.5, but it claimed I was fully up to date, even though I have 5.0 on another machine (which works perfectly).