User interface changes include tabs on top, app tabs (provision for permanent tabs for services such as webmail and music streaming), a combined stop/reload button (like it or not, that seems to be popular among browser developers).
Known issues include slower than usual operation for some users of Twitter and Gmail. plus the inability of beta 7 to work on PowerPC-based Macs.
Beta 7 was released nearly two months behind schedule. The release of beta 8 is still listed for November 12, which now must be extremely unlikely, bringing into question November 26 and December 12 for betas 9 and 10.
If the proposed spacing is achieved, that would push beta 8 to mid January and the release candidates to the first half of March rather than "early 2011" as timetabled.
Firefox 4.0 beta 7 is available here.