I've wanted to install an extension from outside addons.mozilla.org, but Firefox didn't let me do it
So I've did a small research and looks like there are 3 ways to sideload extensions, but all of them suck
Using FF Developer Edition
In the Dev Edition you can set xpinstall.signatures.required to false in about:config, but the problem is that the Dev Edition isn't as stable as standard FF
Temporarily load the extension
In about:debugging#/runtime/this-firefox you can temporarily sideload extensions, but they will be removed next time you open FF, which is annoying
Modify FF code
Lastly, I found this script which modifies the FF code, but this can break things so I don't want to use it
I'm really annoyed that Mozilla gets to decide which extensions I can install. So... what's the best way to sideload extensions?
Edit: thanks everyone, I'm now using a FF fork (Librewolf) which lets me sideload extensions after disabling xpinstall.signatures.required
If you're talking Android, I believe the F-Droid app store (which I'd recommend anyway) packages a version of Firefox (I think called "Fennec F-Droid"?) that allows the same thing as Nightly and Beta (which I assume you mean with option 1).
Also, Beta is very stable, just give it a shot. (Though your extensions might not be, especially on mobile.)
Totally opening up, as I understand it. Which doesn't mean that you can install every extension, because many might not be compatible. For example, Sidebery isn't really useful on mobile, and other extensions (especially the ones still using "Manifest V2") can't deal well with Android unpredictably killing extension processes in the background. But every extension developer can make their extension installable by Firefox for Android users.
People will understand that not everything is compatible, but this is still going to make it much less clunky to run add-ons that aren't readily available now. The previous method of having to create a collection was extremely clunky and often felt like more trouble than it was worth.