My two gripes with Firefox 4 beta is the memory leak which seems to pop up now and then and the delay in having some of my add-ons updated by providers. I miss add-ons like the Evernote add-on which I used almost daily in previous versions. I believe Evernote will only update when a stable Firefox 4 is released and that shouldn’t be too far off, I hope.
As for the memory leak, it seems to affect some of the beta releases and just when I thought it was gone in b10, I saw it again in b11. That said, Firefox still has a smaller memory footprint than Chrome, for the most part, when you add up all the Chrome instances (at least on a Mac).