Firefox 5 is officially out mere months after Firefox 4. It's already available to Ubuntu 11.04 users according to the Launchpad page for Firefox. How's that for fast?
So fire up Update Manager and update today.
This being the first Firefox release on the new release cycle, there's very few changes compared to previous major version releases. Of course, that's still quite a lot going on where the user can't see the changes.
Performance improvements - There's speed improvements every release, but I think most wouldn't even notice it although it's there.
Better standards compliance - HTML5, XHR, MathML, SMIL, and canvas. They've all been improved upon.
No Channel Switcher - The Channel Switcher has been disabled for Firefox 5. It'll make it back later. There's just some issues the developers need to iron out.
There's a list of bugs fixed in Firefox 5 and it's pretty long.
Most Ubuntu users probably wouldn't even notice they'd been upgraded to Firefox 5. It's just that similar to Firefox 4.
Running Firefox nightly build on Ubuntu 11.04
Installing Firefox Aurora on Ubuntu 11.04
Chromium 12 on Ubuntu 11.04