Firefox only got popular because it was so much better than IE at the time.
Now Chrome and Safari are the preinstalled on most devices or are pushed on you if they are not. They are also good enough or better that users don't have enough incentive to switch, especially with ad blockers working just fine on the competition (for now...). And Google won't allow Chrome to fall behind in a meaningful way.
There is not much market opportunity to compete against the monopolistic distribution channels that Apple and Google can leverage.
Firefox only got popular because it was so much better than IE at the time.
Now Chrome and Safari are the preinstalled on most devices or are pushed on you if they are not. They are also good enough or better that users don't have enough incentive to switch, especially with ad blockers working just fine on the competition (for now...). And Google won't allow Chrome to fall behind in a meaningful way.
There is not much market opportunity to compete against the monopolistic distribution channels that Apple and Google can leverage.