Between 29 and 31, Mozilla reworked the basic layers canvas code and this fixed the display problem with Google Maps and a lot of other sites. Plus, there don't appear to be any new regressions with the graphics code switching to Moz2D, and our "blue thumbnails" patch sticks fine in 31, so that's a lot of potential problems avoided and repaired.
The second major bug is that the current version of the QuickTime Enabler does indeed hang up in TenFourFox 29+, as reported by a couple folks earlier. This is a problem with the old Add-on SDK jetpack version in v116, confirmed in that the MacTubes Enabler, which has a later jetpack, doesn't do this. Both of them will need a jetpack update for 31, but I updated the QTE jetpack and added a couple minor bug fixes and released that as QTE v120. You can get it from the QuickTime Enabler wiki page. It will work with 24 and 31 both, and then the QTE and MTE will be refreshed again after 31 is released. So that should be solved.
The two remaining issues are also 29 holdovers and while I consider them important to fix, I don't consider them showstoppers. It does look like editing keys have regressed per your reports, but not all keys have, so I'm not sure what's going on (menu keys and productivity shortcuts, for example, all function). This looks like our bug, but while I intend to fix it I don't consider this a shipping blocker for 31.0. Also, we still have the problem where webcam audio does not work (and neither does video-with-audio). However, our patch to force webcam video and video-without-audio does work, so the webcam can still be used for snapshots as its primary use case, and this is also shippable even if it's suboptimal.
31 also uses Mozilla's new generational garbage collector (GGC), which is even more aggressive about memory than the exact rooting GC in 29. The settings 29 use that we experimentally determined earlier in this blog are tuned for the older conservative garbage collector and may be counterproductive with GGC. I will likely change the time slice and some other parameters for 31 beta, so if you are using these custom GC settings please revert them before upgrading or the browser may not perform well.
Other than that, the browser appears to work fine and I think we're in good shape for another successful stable branch. Once I have the Google Maps problem fixed, I'll grab the beta source when 31 migrates from aurora and we should have a full-spectrum 31 beta release available a few days after the 24.6.0 update. Speaking of, 24.6.0 will be a security update only; there are no bugs on our side that will land, and I am not likely to do further feature work on 24 as the switchover to 31ESR approaches.
By the way, if you've got a newsstand nearby, you might want to pick up the current (June 2014) issue of Mac|Life and their article on rejuvenating your old Mac: