On the normal version of firefox for those platforms you can't permanently sideload an extension, you need to reload it on each restart.
The Developer Edition still lets you sideload addons normally, however.
https://the-pwner224.neocities.org/compiling-firefox/index.h...
This also provides a good security benefit - for extensions that aren't continually updated you can download them with the "Extension source viewer" extension and then change their ID in the manifest and manually sideload them to prevent them from auto updating and using invasive permissions - malicious buyouts by ad companies are not unheard of in both the Chrome and FF app stores.
And also important, you can tweak the extension code as desired. For example the Old Reddit Redirect extension, which automatically switches reddit.com to old.reddit.com also changes new.reddit.com to the old one, so if you ever want to actually visit the new design then you have to disable the addon. There's no way to change this aside from editing the extension and then loading it unsigned.
Are you sure about that?
I am using Firefox on Windows and I currently have no problem sideloading extensions. They stay enabled and even update automatically.
Try, for example, installing this [1] bypass paywall extension via the "Download and install the latest version" link. Works flawless. ... or am I getting something wrong?
[1] https://github.com/iamadamdev/bypass-paywalls-chrome/blob/ma...
Still not great, but it is good to learn that it is just a little less restrictive than I thought.