If you are following what is happening in the mozilla tree you are now aware that Grand Paradiso is out. Is this good are bad news for the users of Camino, Firefox or Thunderbird on Mac OS X. I would say it is more good news than bad ones.
It is good news because with the switch to cairo, we are moving away from Quickdraw. Quickdraw was the initial library for all graphics related stuff on the mac os and it was design in the beginning of the 80's and updated in the 90's. On Mac OS X the graphics got changed twice in the recent years, the first one was named quartz and the second one was named Core Image (nquartz in cairo land), cairo is able to use both - meaning that the GPU sitting on our graphics cards might be used in the future.
If you are not a gecko user, but still a mac user, you should also take the plunge and try out that build. Because Cairo will be used by other project than mozilla [that's one great reason for the switch], trying and reporting bugs in mozilla - might also mean you are reporting bugs against cairo.
The second good news is that at the Rendering engine layout has been simplified, redesigned and that the code is less complex, and should be easier to maintain - old bugs are getting fixed. Mozilla now passes the acid2 test.
What might be the bad news then ?
Well if you have a look at the people doing all those nice development, none of them develops on the Mac , they either use linux or windows. Meaning their most of their test where done on those platforms. And If you want to count the mozilla developers that are working on macs, I believe this can be counted on less than one hand. Meaning that testing for all these new features needs to happen outside - but we also need to make sure that all those bugs get reported, documented properly in bugzilla.
This can easily be done - download Grand Paradiso and use it for one day as you would use Camino or Firefox - report bugs you see.
bz actually got a MacBook or MBP recently; we were helping him out with a few things on #camino not too long ago :)
But your point is still valid; Cairo has been enabled on other platforms for 4-6 months now, so the Mac code needs to play catch-up with all the bug-reporting.