Its only a matter of time now until they decide to start selling Firefox. It doesn't matter if its open source or not, look at XChat for example.
I doubt they'll ever sell Firefox. They would lose their entire fanbase, and they'd be turning their back on the people that made them successful. I don't think they'd do that.
XChat is only sold for Windows, and anybody else is free to download the source, compile, and give it away for free if they want. The reason they sell it for Windows is that it's such a pain to compile it there. If Firefox did something stupid like that, I guarentee somebody would be compiling it/distributing it for free.