(Just checked Wikipedia, apparently several sources pegged them at about 95% share around 2002-2004).
I assume most people install Firefox because it's better then the default browser experience. The argument in the parent comment is that this isn't the case so those that would normally go out of their way to install it have no reason to do so.
e: Or maybe I read it incorrectly? Firefox is a better mobile experience? If that's the case, I'm not really sure what the parent comment's argument is.
Defaults are a big deal.
For the E.U. I quote Wikipedia:
"By European Union law, very large market shares raise a presumption that a company is dominant, which may be rebuttable. If a company has a dominant position, then there is "a special responsibility not to allow its conduct to impair competition on the common market". The lowest yet market share of a company considered "dominant" in the EU was 39.7%." http://en.wikipedia.org/wiki/Monopoly#Law