> i can't just install a program _i wrote myself_ on a _device i own_ without having to get it signed
You're moving the goalposts.
But, because of the fact that you can run unsigned extensions on nightly builds, there is nothing stopping you from distributing your unsigned extension through github or bittorrent or anything else.
The original person you replied to said "Same with a Firefox extension." not a "nightly or developer build of Firefox extension".
If you can't get an extension on a regular build of Firefox, you might as well not be able to do it because for most users installing nightly or dev is a big problem.
You're moving the goalposts.
But, because of the fact that you can run unsigned extensions on nightly builds, there is nothing stopping you from distributing your unsigned extension through github or bittorrent or anything else.