Illumos is also a supported platform (SPARC and x86-64).
What in your opinion would have to be the size of the source code to warrant jumping through the hoops to get this software running, as opposed to a combination of find + xargs + egrep,fgrep,awk?
That's not accurate.
The test suites are not run on CI for tier 2, but they are at least guaranteed to build.
Tier 2 platforms have binary builds, are supported by rustup and often work just fine.
You need rust to build Firefox, so any platform that can't get it to work well is going to be at a disadvantage.