They needed to differentiate themselves years ago. The value prop in the early days was "very light & fast, blocks popups, has tabs, is totally free with no ads". It was a no-brainer to install it on every barely-technical relative's computer a nerd could get their hands on. The only thing that approached it and wasn't
incredibly obscure was Opera, and that either cost money or displayed ads.
FF is no longer "very light & fast" (it may no longer be possible to attain that and actually support modern web browsing—Safari is the only mainstream browser that's even close, AFAIK) and does have ads. Their various interface redesigns have made it confusing as hell to my parents. Old-school popups are handled well enough by ~every browser and they just about all use tabs and have for over a decade. Meanwhile they've added... what? To differentiate them? Plugins. But now those are the same as Chrome's. Very good dev tools—but now those are available elsewhere, too.
They needed to go all-in on something radical years ago to have a long-term shot at relevance. Decentralized social networking or chat built-in to the browser. Aggressive built-in ad blocking & unique user-empowering controls. More, not fewer, non-HTTP Internet protocols built in. Something. And clearly not Pocket.
Now I don't think their market share's big enough for even something like that that to save them. In fact now it'd likely just kill them even faster.