Okay, figured it out. Thank you for giving me the search term "Unbranded Firefox"—I didn't realize this referred to something specific.

Unbranded Firefox is actually a specific version of Firefox distributed by Mozilla, which allows you to disable extension signing requirements. I am very glad to see that they offer this, and I will be using it from now on.

https://wiki.mozilla.org/Add-ons/Extension_Signing#Unbranded...