I recently learned Google Chrome does something similar with browser extensions[1]. You cannot[2] install .crx files that have not been published to the Chrome Web Store.
Sadly Mozilla seems to do something similar with Firefox, likely as another round of copying the most stupid Chrome decisions.
The recently released new "stable" version of Firefox for Android that supports just 9 specific extensions at the moment might be actually even worse than Chrome.
[1] https://blog.chromium.org/2015/05/continuing-to-protect-chro...
[2] Unless you're using Linux or the enterprise version of the browser.