[lapcat]

This is actually no different from Firefox: "The Allow Unsigned Extensions setting resets when you quit Safari; set it again the next time you launch Safari."

You can run an unsigned add-on in regular Firefox by opening about:debugging#/runtime/this-firefox and clicking Load Temporary Add-on...

In both cases, it only lasts until you quit the app.

[TheBrokenRail]

And that's what I deserve for not reading the entire page. Thanks for the correction.