This is disappointing. Everything is becoming centralized, even Firefox extensions. I wish there was an opt out like "unknown sources" in android, but they keep saying we're not smart enough to make or own decisions. They won't even put one in about:config. This change well undoubtedly upset developers and other techy folk, exactly the kind of people you want working with your software.
Fdroid is working on third party repositories, maybe that will catch on to decentralize the mobile world a bit. Something like that for browser extensions would be sweet. Take a look at Fennec Fdroid for a cleaner Firefox mobile experience at least.
The point here is to stop junkware authors (who operate pretend-legally) from trivially installing extensions into Firefox. Right now, this type of software commonly injects javascript into all web-pages a user visits which do things like add adverts or redirect searches.
If you allow a tick box to disable this, then how do you stop the junkware authors from simply checking that box on behalf of the user? Because that's what would happen, the user would click "next" on some random installer (which the junkware authors argue grants them expressed permission to install), and the junkware will claim they tick the unknown sources box to fix a "backwards compatibility issue."
What they're trying to do is make the option to disable the check SO niche that it really isn't a valid option for the junkware authors to use anyway (since most consumers won't have it, only corp. networks which are a hard target for junkware for other reasons).
Fdroid is working on third party repositories, maybe that will catch on to decentralize the mobile world a bit. Something like that for browser extensions would be sweet. Take a look at Fennec Fdroid for a cleaner Firefox mobile experience at least.