Also of note: apparently Thunderbird is receiving enough donations, and has been for a while, to give faith that it will be able to manage just fine independently. Good news IMHO.
About a decade ago, Mozilla decided that Thunderbird should be making a bigger effort to pay for its own funding. The two business opportunities that were implemented were paid email accounts (driven by a survey that found that a substantial portion of new users expected they were getting an email address when they downloaded Thunderbird), and paid support for the large-attachment uploads. Some of the companies involved have since withdrawn the contracts.
In any case, the money being involved (I'm told) isn't stellar--on the order of $10s of K per annum in total.