Since the Firefox installation is an .xpi file, I assume it's also blocked by Mozilla? Not saying I'm surprised, though, giving the nature of the extension.
Not sure, perhaps the author of that one just hasn't tried to get it approved. There's also https://addons.mozilla.org/en-US/firefox/addon/bypass-paywal... and a number of other ones for Firefox that don't look to be blocked, so I don't think Mozilla is taking the same stance.