I'm not sure what you expect to happen. If they don't do this, Russia will ban Firefox. And I do think, it's better for the Russian people to have Firefox available, even if it bends its knee in certain situations. Because I'd wager the alternatives proactively stick their tongue up Putin's.
Ah yeah, true, getting just the signed XPI should work as well.
And well, it is tricky. The signing requirement allows them to block malicious add-ons, which could also be used for state censorship.
I think, offering a separate path for people to install unsigned extensions, if they need it, while blocking them for the majority and therefore making them inviable for malware to target, that's in principle a smart compromise.
Also, side-note: Folks who are on Linux likely don't need to install a separate version of Firefox. Linux distros tend to compile with the unsigned extension support enabled (just need to toggle the flag in about:config).
Mozilla Firefox Blocks Add-Ons to Circumvent Russia Censorship (theintercept.com) en
Finally! Mozilla Firefox is Bringing in Tab Grouping Feature (news.itsfoss.com) en
Mozilla Firefox should gear up as an excellent option for power users and productivity enthusiasts with this feature....