Just looking at the download count from Mozilla for the first 24 hours of Firefox 3, it looks like the browser upgrade is one of the fastest-adopted new program versions ever. Eight million downloads in the first 24 hours can't be wrong, right? Of course, no new program--even one as heavily beta-tested as FF3--is problem-free, and so I've got two fast course corrections for those struggling with incompatible plug-ins and Flash foul-ups.
Let's look at Flash first. Towards the end of my use of Firefox 2, before upgrading to Firefox 3, I noticed a peculiar Flash problem: embedded videos would freeze after two or three seconds. Clicking on the video progress bar would start playback again, only to have it freeze shortly thereafter.
It took some digging on the Flash and Mozilla help forums, where people suggested everything from disabling any ad-blocking plug-ins to tweaking how Firefox handles cookies--all to no avail. Then I struck gold. The solution, as simple as it is, is to simply uninstall Flash 9. I recommend using the Revo Uninstaller.
I can't guarantee that this will work for everybody, but since I was having the problem on my home and work computers and the Flash 10 beta solution fixed it on both, I'm reasonably confident that it should work for most of you.
For most incompatible add-on issues, I'd recommend MR Tech Toolkit or the Nightly Tester Tools. The former is a nearly omnipotent plug-in that can kill the compatibility check for all plug-ins, among its other powers. Nightly Tester Tools removes that check on a case-by-case basis. However, that won't work for all users, and some people need Firefox 2 for only one or two specific tasks.
If you're on a Mac, I'd recommend using the Multi-Firefox app. For PC users, the best bet is to download the portable version of Firefox 2. You can't run both versions simultaneously, but you can have them both installed since they have different profile locations--just quit one before you start the other.