Marketing and marketing budgets has a lot to do with it. Opera supports most of the extensions that are popular on Firefox http://my.opera.com/Rijk/blog/2006/07/04/top-150-popular-fir...
Firefox's mantra of being lightweight also helped them as you could just install the plugins you wanted, not a load of stuff people thought you wanted or should have that could have got rolled in as standard.
There were years between the last good Netscape Navigator and the first usable Mozilla, let alone Firefox 1. I really doubt there was that much loyalty during that long dark spell, especially since during much of that period, Internet Explorer was pretty good.