As Herwig Bauernfeind from Bitwise Works made clear in his presentation he gave at Warpstock 2018 Toronto, Firefox for OS/2 is on its way out for OS/2 after version 52 ESR. The primary reason is because Firefox is switching to RUST. Rust is a general purpose programming language sponsored by Mozilla Research. It is unlikely that RUST will ever be ported to OS/2.
Rust was the primary reason we dropped source parity for TenFourFox also (though there were plenty of other reasons such as changes to the graphics stack, the hard requirement for Skia, Electrolysis and changes to ICU; all of this could have been worked around, but with substantial difficulty, and likely with severe compromises). Now that Firefox 52ESR, the last ESR to not require Rust support, is on its last legs, this marks the final end of "Warpzilla" and Firefox on OS/2. SPARC (and apparently Solaris in general) doesn't have rustc or cargo support either, so this is likely the end of Firefox on any version of Solaris as well. Yes, I use Firefox on my Sun Ultra-3 laptop with Solaris 10. There are probably other minor platforms just hanging on that will wink out and disappear which I haven't yet discovered.
Every platform that dies is a loss to the technical diversity of the Mozilla community, no matter how you choose to put a happy face on it.
If you were trying to get a web browser up on a new platform these days, much as it makes me sick to say it, you'd probably be better off with WebKit rather than wrestle with Rust. Or NetSurf, despite its significant limitations, though I love the fact the project even exists. At least there's Rust for the various forms of PowerPC on Linux, including 64-bit and little-endian, so the Talos II can still run Firefox.
With FPR9 TenFourFox will switch to backporting security updates from Firefox 60ESR, though any last-minute chemspills to 52ESR will of course be reviewed promptly.
UPDATE 7/5: Someone in the discussion on Hacker News found that at least $12,650 was raised by the OS/2 community, and they're going to port a Qt based browser, which means ... WebKit. I told you so.