

One of the advantages of Relay is that it is agnostic of your email provider, making it easier to switch providers without having to change the email on every account that has an alias.
Considering this, I’d be tempted to go with Addy.io instead of ProtonMail / SimpleLogin (subsidiary of Proton AG).
If you’re concerned with having to trust a third-party to process your emails however, Proton may be the better option with built-in aliasing. Mailbox.org is another option recommended by privacy guides with built-in aliasing.
If you’re concerned with Mozilla’s TOS change however, you may also be concerned with the Proton CEO implicitly supporting the current Trump presidency, believing that the Republicans will do a better job reigning big tech in (While I’ll agree that the democrats are not anti-corp, that died with Bernie, I think it’s foolish to believe the republicans will be better). They also pulled their entire media presence on Mastodon, and recently integrated Zoom despite explicitly stating that it has privacy issues in their blog.
I think some people are being a bit extreme in their characterization of Proton AG right now, but it definitely feels like they’re making some peculiar choices when looking at their guiding mission of privacy / security.
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.