FF3 has been pretty stable since Beta4 or 5.
And the memory management is tons better then FF2, especially if you are like me and have a dozen tabs running constantly for days.
Some sites do cause the memory usage to balloon, but normally with 11 to 12 tabs, my machine hovers around 220 MB of usage.
And Zelda, you can force Compatibility override for any extensions you want. Now I just have to remember whether that is a feature of the nightly tester tool, or built right in
GeoffC, its definitely worth getting, especially RC2.