While I was trying to analyze the problem, I did notice that we wait a long time to do garbage collection. Mozilla themselves thought this was a problem on slow systems and FirefoxOS in fact garbage collects at a threshold an order of magnitude less than regular Firefox. Since we're closer in spec to FirefoxOS devices, now we do too. It does seem to cut back on some of the random pauses and stutters after the browser has run for awhile, and it will be in 22.1 final even though it did not help with jemalloc. That said, the experience was valuable in that it looks like issue 231 needs to be enabled for 10.5 as well.
Incidentally, this required me doing bug fixes to Apple gdb to handle frames better. I'll make our hacked gdb-768 available a little later (it's the Xcode 3.0 debugger built for 10.4 with additional patches).