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.
If you can recommend an fork that allows extension sideloading but is kept up to date, please do so, I’ve been looking...