If it's for an entire company, then it's easy enough to compile your own copy of firefox that accepts extensions signed with the company signature rather than mozilla.

It's really not. Small businesses exist.

If you have the resources to develop an internal company addon, you have the resources to build a firefox that accepts a different signature.

Respectfully disagree - having to rebuild each time patches come out, on multiple OSes and versions, which have a patch to allow unsigned extensions is a massively more time expensive than developing a browser extension, and requires extra knowledge on the behalf of the persons responsible

Luckily it's not necessary: you can still enable a flag in ESR releases that allow installation of unsigned add-ons, so that solves it for company-internal tools.