The browser might be ready but the add-ins are not. I just installed Unbuntu 8.04 Hardy and the FF v3 beta 4 worked great, however 70% of my add-ins didn't work. Including things like Filter-set G (must for add blocking), Tab Mix Plus and FoxMark book mark Synchroniser, CustomizeGoogle. Things I can't live without. SO I went back to FF2.
By the way you can have both installed and only minor problems using them with same settings. Don't launch at same time though
Under "tools" in the extensions manager, there will be an option "make all compatible". It'll disable the compatibility checks. Works great for me so far.
I did that but it somewhat defeats the purpose of the whole thing. The problem is that some of the add-ins do NOT work with the beta version so overwriting the check is kind of an exercise in futility