[scriptkiddy]

It's really not though. Try to put yourself in Mozilla's shoes for a moment:

You run arguably the most "free" and "open" internet browser in the business. You are constantly pushing for standardization in the face of competing browsers like Safari, Chrome, and Edge; all of which are trying to establish a walled garden powered by browser specific features. Your browser is consistently losing market share to Chrome because you don't have the cash to spend on marketing partnerships and pre-installs like Google does. Your browser is a means to an end, not the end itself. Your browser exists purely to push forward the tenets of the open internet and open source software. You need market share to push further standardization and improvements to the web experience. To get this market share you need to ensure that your browser offers at least the same performance and security as other browsers. In order to do this, you need to make some backwards incompatible changes or risk falling into obscurity. It's a sacrific you have no choice but to make.

[wtallis]

There's no question that breaking backwards compatibility is eventually necessary. That doesn't mean Mozilla did a good job of handling this transition. They absolutely screwed a lot of their existing users, and without good reason. They have had no overlap between support for WebExtensions and XUL extensions. They've been marking XUL extensions as "Legacy" for months but prohibiting you from installing a WebExtension on anything prior to FF 57, forcing everyone to deal with jarring changes to all of their extensions at the same time.