The implied subscription threat is a complete red herring. They should be ashamed for even mentioning it in their fundraising messages.
Wikipedians wouldn't work for free for a subscription service. The whole project would fork to a new host. The Wikimedia Foundation's own mission statement says, "The Foundation will make and keep useful information from its projects available on the internet free of charge, in perpetuity."
You can't contribute donations to Firefox; you can only contribute to the Mozilla Foundation, which spends most of the money it gets from donations on things that aren't Firefox.
Wikipedians wouldn't work for free for a subscription service. The whole project would fork to a new host. The Wikimedia Foundation's own mission statement says, "The Foundation will make and keep useful information from its projects available on the internet free of charge, in perpetuity."
https://wikimediafoundation.org/about/mission/