I don't get why Firefox gets flak, then.

They broke compatibility with v57 exactly for that: to make continuous small changes. The previous situation was untenable since there was no API for extensions, the extensions could just go through Firefox internals and change at-will whatever they wanted.

The whole point of WebExtensions is that they won't need to break compatibility after this huge leap.

FireFox constantly broke all my extensions with just about every minor update. It was maddening. After being a die-hard FireFox/Netscape/Mozilla user since the 90's, I finally gave up a few years back and switched to Chrome. I can't recall a single time since then when my browser or extensions stopped working because of any update, not just a minor one.

You should maybe check things out again, cause that’s why they made this huge change in Firefox 57.