Any donations you send to Mozilla today go to the corporation and are not spent on the browser. They are spent on things that have nothing to do with the core mission of the maintaining the browser.
Nobody is allowed to fund Mozilla to maintain the browser, which is the actual question you're asking.
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.
The money flow between the for-profit that develops Firefox and the non-profit foundation is one-way: From for-profit to non-profit. This is because it is illegal for the non-profit to give money to the for-profit.
Any donations you make go to the non-profit. They are not used and cannot legally be used for Firefox development.
Edit to add: Mozilla chose this legal structure. Mozilla chooses to disallow donations or payments directly to the for-profit. For example, nothing prevents using a shareware model, where Firefox is free but you can choose to pay for it. And Mozilla chooses to avoid mentioning this structure when accepting donations from you.