I’ll say it again: what Firefox has is a user base, not a future. It’s not unlike Microsoft right now — their success is based on momentum, not new converts. (If Microsoft and Apple were to emerge from the womb today, who would win?)
Firefox’s user base does not have a lot of new growth ahead of it, unless Mozilla does something radical. The reason is that FF’s differentiators are falling by the wayside.
The biggest threat is Chrome. It’s not that Chrome is going to have dramatic market share numbers. But they are the first viable competition that FF has ever had in the “non-bundled browser” space. A market going from “one” to “two” is a big change. So Chrome will capture a lot of growth (and techie mindshare) at FF’s expense.
FF’s second differentiator is that it is seamlessly cross-platform. With Google’s announcement of Chrome for Mac and Linux, this bit of difference is going away. (The stuff is still beta but I expect rapid development.)
Third, as linked above, are extensions. End-users care about these, and until now only FF had a viable ecosystem. I would even argue that FF’s add-ons served something of a “lock-in” function among certain consumers: they might not care about the browser per se, but they love their little weather widget or iTunes controller.
Fourth, Windows 7. It’s not that people love IE. But IE 8 doesn’t suck, and IE 9 will not-suck more. So for the majority of consumers who care about “good enough”, IE is increasingly good-enough-er.