Personally and strictly my own ignorant and informal opinion, Firefox and by extension Mozilla Corp. was doomed the moment it decided to compete with Chrome by being more like Chrome. Firefox built its userbase by being different than the dominant browser, then decided the only way to not die was to be exactly like the dominant browser except with less icky behind-the-scenes privacy behavior.
That was a fundamental mistake. The who's-got-the-biggest-version-number dick-measuring, the constant deprecation of power features and customizability, everything that was intended to beat Chrome in a contest Google couldn't lose is the reason Firefox can't hold onto a decent market share. The 25% of internet users who wanted what Firefox offered ten years ago still want the same thing now, but Firefox decided it was worth losing that 25% trying to chase the 75%, and they lost.
That was a fundamental mistake. The who's-got-the-biggest-version-number dick-measuring, the constant deprecation of power features and customizability, everything that was intended to beat Chrome in a contest Google couldn't lose is the reason Firefox can't hold onto a decent market share. The 25% of internet users who wanted what Firefox offered ten years ago still want the same thing now, but Firefox decided it was worth losing that 25% trying to chase the 75%, and they lost.