This is the confidently incorrect narrative that gets repeated in the comment sections practically every time Mozilla is mentioned but it's every bit as incorrect now as it's been all the previous times.
So here we go again for the millionth time: the big losses of Mozilla market share were approximately during the 2010s. The era of side bets on unique features is approximately the 2020s. The unique features didn't retroactively cause the market share losses of the 2010s.
Moreover, telling the market share story in terms of specific browser features misses the elephant in the room, which is that Google, with the world's most visited page, and a browser that's the installed default on over a billion devices, grew it's market share with a combination of web visability and dominance over the most used mobile platform. Mozilla could triple their budget and have the world's best browser experience, but it wouldn't make much of a dent against distribution defaults.
I would wager that the impact on market share is driven about 97% by Googles distribution advantage and 3% by aligning with user preferences on features and performance. If being a perfect browser led to market dominance, Opera would have already conquered the world back in 2012, but the economics of building a browser aren't always friendly to the good guys.