Beta 9 has been an ... interesting release. Besides a serious blocker that escaped Mozilla's notice causing large portions of some pages to not render at all (bug 623852), and a flickery title bar which is also a Mozilla bug (partially fixed for Tiger in issue 16; see also bug 621762), we are also seeing significantly faster screen display times and the first community statistics for the nanojit.
Well, the tables are turning over. Thanks to user PoLiYa who wrote in to tell me about his fabulous score on SunSpider with his PowerBook G4/1.5GHz and the nanojit turned on: a whopping 2866ms, down from around 6400ms. That's not a typo; his 1.5GHz G4 benched faster than a 2.5GHz G5 on SunSpider with the nanojit. I thought this had to be a mistake, so I got out my iBook/1.33GHz and repeated the test. It was no mistake. On the G4, the nanojit not only worked, it easily cut SunSpider scores in half, and Dromaeo was also about double the speed. User agg23 over at 68KMLA tried it on his G3, and also saw a similar speed improvement, from 16000ms down to around 8900. Try it yourself and post your stats in the comments.
This does have a penalty to pay: since the trace only pays off when it's cached, the browser does indeed cache it to take advantage of it as much as possible, and this in turn causes greater memory pressure. Benchmarks really make the increased memory usage noticible, but even stock Firefox can bloat a little with regular use despite its aggressive garbage collection algorithms. This was never an issue for PowerPC Firefox before, because it never had a nanojit, but now it does. (This does not happen when the nanojit is disabled, which is the default state in b9 right now.)
There is at least one bug in the nanojit I am aware of that can cause crashes or weird behaviour, but it is infrequently triggered and I have it fixed for beta 10 already.
Assuming Mozilla does not come out with a beta 11, beta 10 will be the final TenFourFox beta. I'm not planning to release a release candidate; we don't have the space, and I'm not all that certain b10 will be the last Firefox beta either. If it is, we will follow up b10 with a formal release and welcome the masses to our fold. Tell your friends: we're bringing our beloved old Macs into the next browser generation. More innovations to come.