I don't think Mozilla will ever switch. The reason why they've their own engine and try to keep a decent marketshare (which they do indeed have), is to be able to have a say on web standards, and the web in general.
Having a say ensure there isn't a single voice taking decisions that led us to things like IE6. And we're actually starting to see IE6-ish stuff happening with webkit, as it's nearly the only rendering engine for mobile.
One vendor != good for standards.
Oh also, their engine happens to be pretty close to webkit performance wise.
> I would think one vendor would be great for standards: it is the standard.
Well, IE6 was the standard. See how many sites were "optimized" for IE6 and getting them to render in a browser that supported these pesky underdog W3C standards was bound to fail.
> Out if curiosity, what IE6-ish things do you see WebKit doing?
Not quite the same, but the -webkit CSS prefix that people use without a fallback for other browsers: webkit browsers display the stuff and on e.g. Firefox or Opera it looks broken, because nobody cares to add a -moz or -o prefixed version, or even the non-prefixed version.
It's not one vendor it's just one (open source) implementation. I'd argue this is better for innovation as new features will be quickly ported between the vendor forks and spend more time improving the product than catching up to other implementations.
> Out if curiosity, what IE6-ish things do you see WebKit doing?
You mean like shipping various non-standard features and advertising them to web developers? Not like other engine's aren't doing similar things in various cases (though WebKit on iOS is shipping features Apple is actively opposing ever being standardized, backed by patent threats, which is a bit different from most browser vendors).
But the real issue is not what WebKit is doing, it's what web developers are doing. And the IE6-ish thing web developers are doing with WebKit is creating sites that deliberately only work in WebKit.
It isn't like WebKit is intentionally doing IE6-ish things, it's just that any rendering engine will have certain bugs. Developers try to work around these bugs, and if their is only "one" implementation, they'll start relying on its quirks. A good example in C would be storing a pointer in an int. It made perfect sense at the time, and it allowed people to be lazy. However it doesn't make any sense these days.
Having a say ensure there isn't a single voice taking decisions that led us to things like IE6. And we're actually starting to see IE6-ish stuff happening with webkit, as it's nearly the only rendering engine for mobile.
One vendor != good for standards.
Oh also, their engine happens to be pretty close to webkit performance wise.