and is not prone to exploit only because it is much less used than MSIE.
Two things wrong with this statement. First, FireFox provides far better security than IE, and will continue to do so, because it's not integrated into the very fibre of the OS. Second, even if lower usage were the only reason FireFox is more secure, what difference does it make? Regardless of the reason, it's still more secure! ;)
Give firefox 90% of the market and watch how fast spyware pulls it apart.
See above - it won't happen like that. Or would anyone care to count whether or not FireFox has even 10% of all total security problems? That's what its market share would seem to indicate, but IE still has way more than its proportionate share of security issues.