You can remove the signature requirement on stable by setting `xpinstall.signatures.required` to `false` in your user.js / about:config
(I wrote most the extensions I installed for my own bespoke use, built locally as zip files and installed via "Install Add-on From File...", and I don't have a problem trusting myself.)
I don't think this is is true for the official Mozilla builds (except for Nightly, Beta and unbranded). It's possible that your distro has a custom build that allows the setting. Arch builds Firefox with `--allow-addon-sideload` which could be the culprit.
No promises that that's actually the right flag. I had a rummage around searchfox and it looks like that just enables extensions that have been placed in special directories (whether they must be signed or not is a different flag). There clearly is a setting somewhere though as the unbranded builds exist...
