The thing is, people willing to maintain a fork could contribute to Firefox today, and reduce the development cost, reduce the need for income.
Sure, some people will be more willing to contribute, if it’s a pure grassroots effort, or if they’re left without a browser otherwise, but to just assume that a fork will fix it, that’s wishful thinking.
Normally, the process is:
Having said that, I don’t know what you mean with “graceful”. Desktop environments may involve lots of packages, which may create configuration files in your home directory or get auto-started in your other DEs, so it can be messy.
Something minimal, like LXQt or the various window managers, isn’t going to cause much of a mess, though.
I guess, creating a second user with a separate home-directory, like the other person suggested, would isolate that potential mess…