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 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.
Even just keeping up with all the standards, and contributing to them, is an enormous undertaking at this point in the web's evolution. Let alone UI design, devtools, porting to mobile OSs, etc.