If you build Firefox on 64-bit Linux, *BSD, etc. for your G5, you may want to check out this Talospace article for an upcoming low-level fix especially as we need to ensure big-endian systems work fine with it. The problem never affected OS X Firefox for Power Macs because those builds were only ever 32-bit, and even TenFourFox is 32-bit through and through even on the G5 largely for reasons of Carbon compatibility which we need for some pieces of the widget code. Since this is syndicated on Planet Mozilla let me give a big thanks to Ted Campbell for figuring out the root cause, which turned out to be a long-standing problem I don't think anyone ever noticed before.

I have not decided what to land on TenFourFox FPR17 mostly because this fix took up a fair bit of time; it's possible FPR17 may be a security-only stopgap release. In a related vein, the recent shift to a 4-week cadence for future Firefox releases starting in January will unfortunately increase my workload and may change how I choose to roll out additional features generally. Build day on the G5 is, in fact, literally a day or sometimes close to two (with the G5 in Reduced performance to cut down on fan noise and power consumption it takes about 20 hours to generate all four CPU-optimized releases, plus another 6 hours to regenerate the debug build for development testing; if there are JavaScript changes, I usually kick off a round each on the debug, G4/7450 and G5 builds through the 20,000+ item test suite and this adds another ten hours). Although the build and test process is about 2/3rds automated, it still needs intervention if it goes awry; plus, uploading to SourceForge is currently a manual process, and of course the documentation doesn't write itself. I don't have any easy means of cross-building TenFourFox on the Talos II (which, by the way, with dual 4-core CPUs for 32 threads builds Firefox in about half an hour), so I need to figure out how to balance this additional time requirement with the time I personally have available. While I do intend to continue supporting TenFourFox for those occasions I need to use a Power Mac, this Talos II is undeniably my daily driver, and fixing bugs in the mainline Firefox build I use every day is unavoidably a higher priority.

http://tenfourfox.blogspot.com/2019/09/a-quick-note-for-64-bit-powerpc-firefox.html