% ./mach jstests --args "--no-ion --no-baseline --blinterp-eager --regexp-warmup-threshold=0" -F -j24
[43359| 0| 0| 614] 100% ======================================>| 529.7sPASS
[43359| 0| 0| 614] 100% ======================================>| 499.0sPASS
[8193| 0| 0| 0] 100% ==========================================>| 132.3sPASSED ALL
[8193| 0| 0| 0] 100% ==========================================>| 133.3sPASSED ALL
Currently the code generator makes heavy use of POWER9-specific instructions, as well as VSX to make efficient use of the FPU. There are secondary goals of little-endian POWER8 and big-endian support (including pre-OpenPOWER so your G5 can play too), but these weren't necessary for the MVP, and we'd need someone actually willing to maintain those since I don't run Linux on my G5 or my POWER6 and I don't run any of my OpenPOWER systems big. While we welcome patches for them, they won't hold up primary support for POWER9 little-endian, which is currently the only "tier 1" platform. I note parenthetically this should also work on LE Power10 but as a matter of policy I'm not going to allow any special support for the architecture until IBM gets off their corporate rear end and actually releases the firmware source code. No free work for a chip that isn't!
You should be able to build a JIT-enabled Firefox 86 off of what's in the Github tree now, but my current goal is to pull it up to 91ESR so that it can be issued as patches against a stable branch of Firefox. These patches will be part of my ongoing future status updates for Firefox on OpenPOWER (yes, you'll need to build it yourself, though I'm pondering setting up a Fedora copr at some point). The next phase will be getting Baseline Compiler passing everything, which should be largely done already because of the existing Baseline Interpreter and Wasm support, and then the final Ion JIT stage, which still needs a lot of work. We'll most likely set up a separate tree for it so you can help (ahem). No promises right now but I'd like to see the completed JIT reach the Firefox source tree in time for the next ESR, which is Firefox 102. That's more than you can say for Chrome/Chromium, which so far has refused to accept OpenPOWER-specific work at all.