Happy as I am to be using Firefox 3.0, as a web designer I felt it was necessary to hang onto Firefox 2.0 for testing purposes, at least for the short term. When installing FF3, it automatically updates your old installation, and there is no way to specify the directory it installs to. So, it was a relatively simple solution to go just switch the name of the FF2 folder to (what else?) Mozilla Firefox 2.0 which cleared the way for FF3 to install to Mozilla Firefox and voila - two versions can happily co-exist.

In windows (before you install FF3):
My Computer>Local Disk>Program Files>Mozilla Firefox <--- change this to Mozilla Firefox 2.0

On a mac, you can simply download the new FF3 application to your desktop, change the name to Firefox 3.0 and then copy it into your Applications folder.

Ok, what are you waiting for? Go and get Firefox 3.0:
http://www.mozilla.com/en-US/firefox/

And, while you're at it - try these handy add-ons:
Adblock plus - don't ever have to see ads again!
ColorZilla - advanced eyedropper tool
Del.icio.us - access and organize your bookmarks
Foxmarks - sync your FF bookmarks across many computers!
MeasureIt - on screen ruler to get pixel width & height of any element on a webpage
PDF Download - choose what you want to do with a PDF file
Web Designer's Toolbar - lots of handy tools in one easy place.