Perhaps because Firefox doesn't support real-world web standards
Firefox is getting better, Opera doesnt come close. Who cares about web standards somebody made up? I only care about the ones in use, not what is on the ever exciting pages of some self appointed guardiens of making sure that IE is doesnt dominant.
Personally, life would be a heck of a lot nicer if the other browsers would go away and all we where left with is the IE Browser and Alternates. For every hour I spend designing a page, a complete complex page that is, I spend up to four trying to get it to work in Netscape and variations, and 12 trying to get it to work in Opera.
While that has more to do with the fact that I think IE first when scripting, the fact is that why should anyone really care about a limited percentage of the population when catering to them takes up way more time than it does to cater to the majority. I have very standard pages for most of my sites, and the percentage of IE browsers runs from 96-98 percent. Opera is hardly seen (and I check with more than just the UA), and Netscape and offshoots amount to less than one percent. But that tiny percent can cost me a weeks on production.
Personally, I dont even bother with Opera anymore, they dont support invisible iframes from what I can tell, and with out that they are useless to me.