People don't move abroad easily, particularly very experienced ones with the historical and institutional knowledge that is really key for a project the size of Firefox - uprooting their families to a foreign (and likely not even English-speaking) country, is a difficult choice. This goes double for Bay Area / SV people used to massive salaries that are hard to justify and achieve elsewhere, and people moving to Europe where immigration bureaucracy is not trivial. Would you take a pay cut and force your family to emigrate just to keep working on a browser, when you've probably plenty of easier and more rewarding options?
As for remote, the political support to fund a project effectively based in the US would be zero.
You also forget that EU institutions don't typically enjoy the freedom to spend a few hundred millions on the spot - they have processes and rules and parliamentary oversight to ensure agreement between 27 governments. It's not like the US president cutting a check, more like the US Congress approving funds - to get any significant amount, it would take some time.
I'm not saying it's impossible, but it would realistically be "next year" (at least) rather than "next week".
I think remote work is missing the point here. It's politically difficult for governments to spend their tax revenue by paying a largely foreign workforce.
Not just politically difficult, it's also generally unwise. In the event of deteriorating relationships, you'll suddenly lose access to what you've been funding, because the US would declare it "a matter of national security" and outlaw the export. You can still work with the sources you have, but it'd be hard to keep it running smoothly. Much better to not have that risk if you're going to spend billions on it.
A) The people with the knowledge just have been fired and are free to move and to be hired
B) Thanks to corona, remote work is a fully accepted option
Something new can be running next week or even tomorrow.