Linux may be largely standardised at the CLI end of the stack - but it's not at the higher up application level i.e. gtk vs qt, gnome vs KDE - and of course the likes of Android, with a completely different non-X model.
And in fact, at the level where it is commoditised, there's a lot that is shared with the commercial Unixes and BSD.
Even at the low level, within the kernel, there are often multiple projects covering single features.
Which is a way of saying that standardising on Firefox OR WebKit would be a bad idea. Would we have even seen the iPhone or Android without WebKit? Difficult to say, as Apple and Google would have put effort into improving Firefox, but my understanding is that KHTML was a far better starting point. And of course, KHTML was a Linux project.
But more to the point, we benefit from having healthy competition, even in free software. The 'one true project' approach presumes that the one true project is going to get it right.
(We can now see in retrospect that XUL was a distracting waste of effort).
As for Opera - which seems to have loads of fanbois - my Google analytics say barely anyone hitting our site is using it - in fact, barely any usage from Nokia/Symbian, or Windows Mobile at all - almost all mobile usage is coming from iOS or Android. Which shows the difference between installed base and usage.
(Mind you we're pushing Android users in the direction of Opera due to Google's SVG fail with their webkit build)