It all started with version 37 when they decided that software freedoms didn't matter and only allowed users to install/use extensions approved by Mozilla. Then dropping their entire extension ecosystem and burning it to the ground made everyone realize what was happening. Now there is no XUL, 1/20th the amount of add-ons, and the ones that do exist don't have the power to restore features of the GUI Mozilla removes. All that's left is a relatively quick (but crashy, because low level features) chrome-clone-in-spirit.
It's also very difficult to keep two separate Firefox installations from interfering with each other, so a lot of people like me switched to using Chrome for 90% of their browsing and kept an old version of Firefox for some use case they couldn't do without.
