Ah, feels like they're following Chrome's example, which decreed that it should be exceedingly difficult for Windows Chrome users to install extensions from somewhere other than https://chrome.google.com/webstore/ . This basically killed an internal app we had at work (a fork of a "REST client", with some added request-signing features specific to our internal APIs.) There was no strong reason to keep it secret, but there had previously been no need to put it in the store either, and there was a $5 charge to publish in the Web Store, which I didn't feel like dealing with.
Anyway, they are both measures taken to stop malware, by taking an option away from the user, that most users won't even notice, but many "power users" will be inconvenienced to varying degrees. I'm guessing Firefox's won't be as bad, since the "developer version" that will let you keep doing the old way probably won't differ from the normal version as much as Chrome's does.
This is the exact reason why I moved to Firefox from Chrome back when Google started tightening the noose around developer mode extensions. I had written a few extensions for my own personal use and had no interest in putting them up in the Chrome Web Store. This was fine and good until Google decided it was A Bad Thing and Chrome started popping up annoying warning windows on every startup and then eventually disabled my extensions entirely.
I switched to Firefox since it let me have more control over my own browsing experience (and gave me a good excuse to extract myself just a little bit from the Google hivemind). I'm extremely annoyed to see that Firefox is now going down this route too.
Anyway, they are both measures taken to stop malware, by taking an option away from the user, that most users won't even notice, but many "power users" will be inconvenienced to varying degrees. I'm guessing Firefox's won't be as bad, since the "developer version" that will let you keep doing the old way probably won't differ from the normal version as much as Chrome's does.