Fortunately, that won't happen to us, at least not yet. Most of the 10.5 compatibility code has been restored successfully (and backported to 10.4, of course), and I have not found any major bugs in this version except for a strange problem with favicons I'm not able to fully track down yet. Now that we're running on a current compiler for a change, I was able to jettison all the ugly fixes I had to make for gcc 4.0.1 and make our changesets smaller and easier to maintain. When 19 goes into beta I'm going to pull it again and possibly generate some opt builds for people to play with, starting our new unstable branch off right. I'm also going to begin work on our PowerPC IonMonkey port and let's hope we get it done before Mozilla sends JaegerMonkey to that great tracing compiler in the sky. Remember: no plugin support exists at all in 18+.
For the stable branch, we will issue a fix for issue 194, which is a Mozilla bug actually but that doesn't appear is being backported to ESR17, and I'm looking into issue 195 where the check for dud fonts introduced with issue 171 can slow down the browser when it gets run unnecessarily, though this probably won't be fixed in time for 17.0.2, our "Best Commodore 64 Monitor Ever" release.