I've been giving specifically to support Thunderbird development, not other Mozilla work [0] for the past few years because I think it's vitally important that email remains an open protocol, supported by mainstream desktop clients. Without Thunderbird, I fear email will be lost to the Gmail & Apple walled garden.
We hope everyone will resume via https://give.thunderbird.net
Feels like an odd choice (and perhaps, an important omission?) when the announcement calls out the importance of donations to Thunderbird.
You'd be donating to a nonprofit and getting the tax benefit, then the nonprofit would be passing it onto a for-profit corporation. End result: tax-deductible donation to a for-profit corp. That doesn't sound like something the IRS would like.
It sounds like the ability to directly generate revenue was seen as outweighing the benefit of allowing tax-deductible donations. (This is my own speculation.)
Would there be a limit? I don't think so. If I was running, say, a non-profit website host, 100% of my raised donations would be going, as expenses, to a for-profit company (some commercial cloud-hosting provider.) That's still an entirely-legitimate model for a non-profit to operate under.