The latest development in the FireFox camp has shown there's, errr... still a lot of development going on and not enough time for everything.
Mike Beltzner, Director of Firefox, said on the Google groups mozilla.dev.planning usergroup, "I have decided that we will be delaying the upcoming code freeze for Firefox 4 Beta 6, originally scheduled to be tomorrow", in other words, the upcoming Firefox 4 Beta 6 will be slightly delayed with a best-case ETA of September the 12th.
Beltzner presented a rather long list of bugs that still need blocking here. One that will require serious overtime and testing to make sure that Beta 6 is the code-freeze release.
After a freeze, development goes into its final stages, usually meaning a final polish on the code after stamping out bugs for final release. In this case, the critical Jaegermonkey engine that will have to challenge IE, Chrome and even Opera (which has made some serious advances thanks to the infamous Browser Choice dialogue in Windows).
Of note is that Beltzner has advised that some planned features for FireFox 4 will be dropped for future releases, a matter he will be looking into personally.