A conservative guess would be around 60 people.

bugzilla.mozilla.org/describecomponents.cgi

You can click around and see the bug reports they’re working on. There are a few, to say the least.

www.firefox.com/en-US/releases/

This is a way to see what’s in each release. The ones on the left are major releases and tend to have bigger features, and the others tend to be bug fixes.

Web browsers start with core functionality that’s very complex. Then you tack on that they’re being used for things like banking, and managing the critical details of people’s lives. That means security galore, which is hard and constant. Then you have ad people, who are also something that’s hard to defend against.
Then there’s the constant flood of new features you have to implement to keep up with Google.

Chrome has 1,000 to 4,000 people working on it. Mozzila employs about 700 to work on firefox, with maybe 1,000 additional open source developers.

My initial guess was very wrong.