What are your goals?
I would say it’s really a combination of the instances policies and their jurisdiction, and in terms of jurisdiction it also depends on where you live (e.g. you may have more protections under law if the instance is hosted in your country)
There’s also nothing stopping you from using multiple instances — siloing your interaction in different types of communities in different accounts on different instances. This may be useful if part of your privacy concerns are having all of your post / comment data on one account on one instance.
Edit: You can also use an email aliasing service to avoid even giving your email out. There are aliasing services such as Addy.io, Simplelogin (subsidiary of Proton AG), Firefox Relay (Mozilla), as well as some email providers which provide (iCloud, Proton, Mailbox.org to name a few)
Normally I would say community forks have the power to continue the project. However, in this case I think chrome / safari would eventually add enough new features that Firefox forks can’t add quick enough. Mozilla at least has some power in pushing the direction of web-standards, which these forks would lack, as well as the larger development team and some corporate usage of the browser which Mozilla has. I also don’t see the smaller development community keeping up with security issues found in the browser, particularly pertinent for corporate marketshare and individuals with a stricter threat model (journalists, dissidents, etc.)
The only other factor, is whether Firefox dissapearing would officially create impetus for an anti-trust case against Google. I doubt so under the current American presidency, but I could see the EU being concerned (even if they lack the power the US has to force the company to split). If something were to happen here there would be substantial change in the browser market, but I wouldn’t be too hopeful of this happening.