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.
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.