I'm not sure what screen "Developer Options" is referring to, but you can load add-ons directly from your hard drive with no fuss from the Add-ons page (though you must be running the Nightly or Developer version of Firefox). Click the gear icon right above your list of installed add-ons (this is also the menu that lets you disable auto-updates).
I can see why you'd think that but in practice I assure you that your concern is unwarranted. I've been using Nightly Firefox exclusively for almost ten years and I honestly can't remember it ever crashing (excluding the times when I was manually futzing with experimental about:config flags back in the electrolysis days).
As for the developer edition, it's literally the version that they expect web developers to use; it's not half-baked software by any means.
The Developer Edition is not a nightly build, it’s a beta build, so there has been some testing (Before I switched to stable, I only once had an issue). Your point stands though.
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...
I'm not sure what screen "Developer Options" is referring to, but you can load add-ons directly from your hard drive with no fuss from the Add-ons page (though you must be running the Nightly or Developer version of Firefox). Click the gear icon right above your list of installed add-ons (this is also the menu that lets you disable auto-updates).