Yes, FF was revelatory (features and performance) and, relatively, very popular for a time. 31% was a massive share considering it was up against a browser that was the default for the vast majority of people using computers.

Mozilla have had so many chances to position themselves as the privacy-preserving alternative in current years but just can't get out of its own way in any sense (e.g. corporate greed or being hostile towards users). There's still dim hope for FF and some of its forks, like Librewolf, but hopefully forward thinking projects like Servo and Ladybird can fill the void.