lack of investment in gecko and dropping marketshare of firefox will result in more and more compatibility issues over time (which further accelerates dropping marketshare), until they're eventually forced to become another chromium based browser
Now Firefox is moving into that role. Except Firefox has no killer captive audience. Safari was pushed because of iOS Mobile users. Firefox doesn't have that.
So when you're a frontend dev at big corp, and you have to get stuff done now, targeting the quirks of a browser used by less than a tenthbof a percent of your userbase doesn't factor into the equation