Mozilla are doing other things that a browser, yes. And this is good. Browsers are special and don't make money by themselves, and Firefox in particular is entirely dependent on Google's money. Having alternative projects that can bring revenue (e.g. Pocket) helps them remove that singular dependency and ensure they can survive long term.

And having a specific "donate to Firefox only" would probably end in disaster. They might end up in a situation where they're forced to waste money on Firefox because that's what the donations are for while not having enough money to keep the lights on in offices. For a fun example of what happens when you have fixed budgets that don't have any flexibility, Atlanta's MARTA was founded with an agreement providing public funding, with a fixed 50/50 split between capex and opex. So they found themselves with brand new trains because there's capex budget to spend, but falling apart infrastructure because 50% wasn't enough for opex.