Not sure how reliable a source it is, but Wikipedia cites a Mozilla engineer's tweet as saying they have 600 employees, which is a bigger operation than I would've guessed: https://twitter.com/#!/paulrouget/status/116110841669099520

they doubled size this year to counter chrome's speed of development

The 64M number is for everything: browser development, website, various infrastructure (direct hosting costs, development, maintenance) like bugzilla, the addons site, the update servers, and so forth. Also for legal, marketing, HR, QA, etc.

300 people is a very low estimate for every single browser development effort on the market right now. For example, Opera had over 700 employees in Feb 2011 according to <http://my.opera.com/haavard/blog/2011/02/01/decade>. I'd be incredibly surprised if Google doesn't have at least several hundred people on just the non-WebKit parts of Chrome, plus the people working on WebKit at Google, Apple, and other places. Microsoft had somewhere between 30 and 50 people working on just the JS engine in IE9, according to unsubstantiated rumors, for what those are worth.

So the short of it is, browsers are harder and take more effort to develop than most people think. ;)