Well, according to the Mozilla foundation this release includes few bug fixes such as ...
*. Fixed some issues with the Adobe Flash player under Mac OS X.
*. Few enhancements concerning the memory (RAM) usages (well ... I don't know about you, but think I'm gonna upgrade :D ).
*. Graphic driver comp-ability fixes.
*. Few GMAIL slow loading bug fixes.
Now these are just the main ones since they have patched a long list of errors. Anyway, remember that, since still Firefox4 is in beta stage, if you're a Hulu.com fan (oooh no what is it now....), then they still not displaying content (not for this release but has been around for the whole Firefox "4" version.
So if you want to install it fresh or wanna upgrade then click here for the link.