10.4 will not build Electrolysis without writing support for issue 66, and based on some code I've been reviewing, possibly not even then. There are lots of workarounds for threading problems in 10.6 which will almost certainly plague 10.4 and 10.5, quite probably other problems with the older operating systems, and then potential endian issues with serializing data over IPC. If Mozilla had gotten e10s working for Firefox 4 as originally planned, there was a good chance we could have made it work then since Mozilla still supported 10.5 and most of the code was still appropriately endian-agnostic, but they didn't, and we won't. 10.6 support is likely to end in the very near future as well, which dooms us further.
As a result, I'm declaring now that in its current state, Electrolysis will not be supported by TenFourFox. I don't think it can be done on our older OSes in a satisfactory fashion, let alone by one guy, so if and when Mozilla makes it mandatory that's where source parity will end. Assuming no breathtaking technical breakthrough on my part, if it appears to be imminent after the 38ESR diversion (which is my current estimate), we go to feature parity at the end of 38ESR.
Remember, feature parity does not mean the browser stops developing -- it just stops mirroring an official branch of Firefox. By 38 we will have the strongest suite of HTML5 and ECMA6 support available for OS X on PowerPC, including <picture> support and WOFF2 webfonts, and I'm still working on getting IonMonkey PowerPC complete as well as our MP3 playback module. Should I cut support there, I also intend to continue backporting relevant security and stability fixes to the extent they apply, as well as necessary updates to our encryption suite so that HTTP/2 and TLSv1.3 are fully supported. Suitably maintained, we should still have a top-tier layout engine for at least one to two years after the end of ESR38, and a functional one for some time after that. Watch this space.
The release of 31.3.0 got delayed to December 2, which is annoying, because I spent the weekend building it instead of watching the calendar. Fortunately, no patches relevant to us so far have landed afterward, so these already-built versions will be uploaded later this week assuming there are no last minute changes. Happy Thanksgiving.