Donations to for profit organizations are not tax deductible and have to be reported as income. If the Mozilla Corp accepted donations there would likely be a lot of confusion about whether the donations could be written off for taxes as donations to Mozilla Foundation. There’s also potentially complications with state laws around solicitation.

If they wanted to allow donations they would have to be really clear that it’s not tax deductible. They would also have to give a cut to Apple and Google if they accepted donations on their mobile platforms.