Earlier quoted context omitted.
> provide an XPI (that will only benefit the few since the side-loading process is made awkwarder at every point release) ? This add-on is the first XPI I have ever loaded. I didn't find it awkward to do; it is simply: Tools -> Add-Ons -> Cog Icon -> Install add-on from file... The only thing I found unintuitive was the cog icon.
Have you restarted Firefox since you did that? Can't test right now, but I'm pretty sure it will be uninstalled, by design.
Mind you I'm using Firefox 61 (current version is 63). I switched off auto-updates when it kept changing my user interface and trashing the extensions I need to use to make the web half-usable (e.g. Adblock Plus, Bypass Paywalls, Disable Javascript, Font Contrast Fix, Stylish).
I'm guessing Firefox 63 doesn't let you run Bypass Paywalls, then? I suspect if fucks up other stuff, though I'm not trying it to find out.
If Firefox want to increase their market share, a good start would be not to make changes that are actively hostile to the user.