Provided useful features and functionality that people wanted. Tabs and plugins are a couple of big ones off the top of my head that people loved. It was also adopting open standards and new technologies though, which was an uphill battle because Microsoft was fighting them with incompatibilities and closed extensions, but people actually did like those things. Good support for developer features as far as I know went a long way to getting around that and having content providers take up and support firefox as well although I'm not a web developer so I'm a bit less sure of that aspect of it.

Competition has improved but so has firefox. Why should the expectation be that they stood still while everybody else went ahead? And it absolutely has declined. You also did say that they don't have features you want and seemed to imply that was part of the realities of not having enough resources or struggling to compete with google. Just doesn't seem like the reason holds water.