For the last two days I have been running Beta 2 of Firefox 3. Looks-wise it is almost identical to Firefox 2. Everything (with 1 exception) works the same as before and I don't notice any difference between the two. The only thing is that it feels a bit faster, but, nothing measurable just a feel of faster. The only problem area is Yahoo mail in that it has problems reading my mail, but, I can always go back to version 2. When I get time I will log the whole session and pass the info to the development team for them to review.
The Java based game Runescape works without a problem. I will be trying out a number of other sites and see how it works.
If you want more information you can read MozillaZine.