I don't have any input on direct user funding for Firefox, but Thunderbird is also developed by a for-profit entity and accepts direct user funding with no charitable tax deductions as well. [0] https://www.thunderbird.net/en-US/donate/
Exactly, and to my knowledge the receiving party needs to pay profit tax on them. It's called a donation, but technically more of a pay-what-you-want model. Several businesses do that.
