Mozilla announced yesterday that Firefox will drop support for sideloaded extensions in 2020. The organization said this change, which is set to be finalized with the release of Firefox 74, will "give users more control over their extensions."
As explained by Mozilla's announcement, sideloading is "a method of installing an extension in Firefox by adding an extension file to a special location using an executable application installer." That almost makes the feature sound so boring that it's hard to care about it going away.
Sideloading extensions can be useful, though, because they make it easy to install an extension across multiple instances of Firefox on a single device. Mozilla also said in its official documentation on sideloading that the feature can be used to install early versions of extensions that developers want to test before publicly releasing them.
But those uses don't appear to outweigh the negatives aspects of sideloading. Mozilla said that sideloaded extensions "frequently cause issues for users since they did not explicitly choose to install them and are unable to remove them from the Add-ons Manager." The feature's also said to have been used to distribute malware.
Firefox 73 will start the transition away from sideloaded extension support. Mozilla said that release (February 11 for the general public) "will continue to read sideloaded files, but they will be copied over to the user's individual profile and installed as regular add-ons." That way Firefox users shouldn't have to worry about extensions they rely on mysteriously vanishing.
Firefox 74, which will be publicly released on March 10, will then drop support for sideloading entirely. This timeline should give extension-makers enough time to modify their workflows accordingly.
FF gradually loses customization ability with each version while pretending that it improves security (actually, tries to protect users from themselves) or speed (yeah, checking for presence of userChrome.css greatly slows down FF startup while all their telemetry isn't).
Why they even think that dumb users that can't remove an extension use Firefox? Such users are all on Chrome!