> They made ONE switch to a superior and standard extension model

No, they made TWO. The original model was the XUL-based one. Then there was the Add-on SDK:

https://developer.mozilla.org/en-US/docs/Archive/Add-ons/Add...

THEN WebExtensions.

So some extensions had to be ported twice, within just a couple of years. This alienated a LOT of add-on developers.