I'm fairly sure the revenue attributable to Firefox dwarfs what any of the bigger competitors are spending on their browsers, so it's not really for lack of resources if Firefox falls behind. But it's the cash cow that subsidizes pretty much everything Mozilla does, some of which is great and some dubious. Everyone can make their own case for which projects are great and which are iffy.
Personally, I would think an e-mail client is the perfect complement to browser development, and something that could/should be another significant source of revenue instead of a burden.
> I'm fairly sure the revenue attributable to Firefox dwarfs what any of the bigger competitors are spending on their browsers
I suspect you would be wrong at least for the cases of Google and Microsoft. I don't have a good feel for how many people Apple has working on Safari and WebKit, but both the Chrome and IE teams are significantly bigger than the Firefox team from what I can tell, and probably more expensive unless you think Google and Microsoft pay developers less than Mozilla does. Let me ask you this: how many people do you think Mozilla, Microsoft, and Google each have working on their respective browsers? Ballpark figures, of course.
Also, estimates are that the money Google spends annually just _advertising_ Chrome in the last few years (TV ad campaigns with Justin Bieber and Lady Gaga, worldwide ads on public transit, etc) is comparable to the entire annual revenue attributable to Firefox.
Personally, I would think an e-mail client is the perfect complement to browser development, and something that could/should be another significant source of revenue instead of a burden.