Extensions in Firefox are required to be signed by Mozilla. If you make your own build of an open-source extension, it will not load. The setting to disable this check only works in Developer Edition, ESR and Nightly builds.
Firefox nightly is pretty darn stable, though I'm not sure if there's any security issues with it. I use it for both mobile and desktop, and have for the past 5~10 years and haven't had any issues that weren't fixed by immediately downloading a fresh build (this happened twice) or were caused by me performing unsupported/insane actions (transferring browser profiles (indexeddb) on mobile via CLI commands using remote devtools).
I've got one buggy device, but I suspect corruption due to the error message and I'm more interested in saving the unsyncable data I've got on that profile than I am in trying to fix it by clearing data.
https://support.mozilla.org/en-US/kb/add-on-signing-in-firef...