Installing extensions from a file is supported in the latest mainline FF (84.0.2), nightly or dev are not required. I currently have one installed. It just shows a confirmation dialog and then installs it.
This is true but misleading: the extension you install from file has to be signed by Mozilla in exactly the same way that extensions on the store are signed.
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...
