I'm now going to try Firefox3 (Beta5) - without affecting my Firefox2 installation (using Portable Edition)

I've waited, read a bit, waited a bit more, and now I think it's time to try out the new version.... I'm cautious by nature, and when I found out that it's possible to install Firefox3Beta without affecting my "stable" version 2 (yes, I'm referring to the version that sometimes guzzles up CPU and RAM!}, I decided to follow that route.

Thanks for this info go to Gina: http://lifehacker.com/376551/the-complete-field-guide-to-testing-firefox-3

More specifically: From http://lifehacker.com/355973/make-your-extensions-work-with-the-firefox-3-beta to manually edit Firefox 3's configuration to run extensions no matter what.

Mozilla Firefox, Portable Edition 3 Beta 4 can be downloaded from here: http://portableapps.com/apps/internet/firefox_portable/test

Your Firefox 2 settings can be copied by following these instructions: http://portableapps.com/apps/internet/firefox_portable/test

When you are ready to take the plunge, then download Firefox3 beta from here: http://www.mozilla.com/en-US/firefox/all-beta.html

Moving from Opera to Firefox.... Here's the list of addons that I needed to install

I have just been through the same process (changing from Opera to Firefox).

Thanks to Tokei from http://stephan-mann.de/?p=5 for his article.
In addition to the addons that he listed, here are some gaps that I found and how I filled them:

Extended Statusbar 1.2.7: A Statusbar with Speed, Percentage, Time and loaded size (like Opera's one)

ImgLikeOpera 0.6.15: Allows load only the images that you want

keyconfig 20060828.1: Rebind your keys so that your Opera keystrokes can be used

PageZoom 0.3.3: Adds page zoom functionality.

Secure Login 0.9.1: A login extension similar to Opera's Wand login

Tab Mix Plus 0.3.6: Tab browsing with an added boost.

User Agent Switcher 0.6.10: Adds a menu and a toolbar button to switch the user agent of the browser.

wmlbrowser 0.7.17: Display WML (Wireless Markup Language) content (for WAP sites)

Duplicate tabs: duplicatetabs

Once you have configured FireFox - you don't really want to do it again if your PC crashes - so backup your Firefox addon config using: mozbackup-portable-firefox-and-thunderbird-backup-tool

Here are more sites with interesting info and comments on the subject:
10-features-youll-find-only-in-opera
50-best-firefox-extensions-for-power-surfing