This.

Brendan Eich was the Director of Mozilla. This is the guy who invented Javascript in 10 days, at Netscape, and then co-founded Mozilla, and became the technical lead. He was Chief Architect of Mozilla, then CTO of Mozilla Corporation, then CEO. He made Firefox great. This was when Mozilla was in its heyday, and passed IE in marketshare.

Then he was fired in 2014 because a bunch of people went crazy that he made a $1,000 political donation for a California ballot proposition that had nothing to do with computers.

This sent a signal that Mozilla doesn't reward technical improvements to its software — it rewards following political trends.

All of the bad stuff in Firefox started then.