A good counterexample to his argument is Firefox itself.
(edit: although I do see HTML5 as being a potential threat to firefox and open source browsers in general if h.264 becomes the dominant codec for <video>, but this is another matter)
Firefox is currently the most popular, yes, but Chrome's been growing (slightly) faster and it's still in beta, not to mention it has the full support of Google behind it. How profitable would Mozilla be if it wasn't being paid by Google? (That's not snark, that's a genuine question that I don't know the answer to.)
You can't offer Firefox as a counterargument when his point is that soon Firefox will begin to slip. Right now, it's arguably at its peak. It's competing with two browsers that really are seen as experimental among Windows users: Chrome is still in beta and doesn't support several things; Safari really only got production-ready on Windows with its latest version, and Apple hasn't focused much on promoting it yet. Firefox, meanwhile, has 6 years' community behind it, and inertia alone states that it'll spread as Internet Explorer disappears, and IE seems to be quickly dwindling.
The real thing to check is what the early adapters are using: Not the pure experimentalists but the people who are "browser enthusiasts", the people who recommend browsers to other people. Three years ago they were all recommending Firefox, myself included, and now we see Firefox is the dominant browser. Now, however, all the adapters love Chrome, and the elitist Mac snobs have shifted from loving Firefox to praising the much more native Safari 4, and for good reason. If we assume "a while from now" means 5 years, his argument is correct: Mozilla can't afford to compete with a company as talented, merciless, and rich as Google.
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Which is fine, as long as everyone recognizes it as such. His prediction and reasons for it may turn out to be true, but there's no way now to know if that's the case.
Firefox has good support by developers and I don't see anything changing until another browser becomes just as extensible and achieves the same developer niche. Not to say another browser won't take over the mass market though.
Completely subjective, but I have always loved the Firefox and Thunderbird logos. I've had them somewhere prominent on my desktops for so long now that I almost don't feel right without them. First app that I install on a new machine is Firefox. Maybe Google or Apple should try and snag Jon Hicks, I read he's working with Opera at the moment.