LibreWolf is a decent alternative. I switched to it a while ago as Firefox enshittification required more and more tweaks in configuration to close leaks.
I’ve heard good things about Mullvad browser too especially on fingerprint resistance, but LibreWolf works for me well enough to not search for alternatives.
For rare sites that I need to use and which don’t work in Firefox based browsers, I just use Brave.
Fennec is a poor alternative because it connects to Firefox services. Sync is optional, but some internal components will talk to Mozilla, and Mozilla changed their mind about “never selling your data” recently.
Brave is Chrome with a history of suspicious moves, toxic leadership, involvement with crypto and AI