Regarding strictly cpu/power consumption ratio I guess the mac mini is pretty good.

Use Vanadium. On android, any Firefox based is lacking either in the sandbox or fingerprinting mitigation, plus extentions make you more fingerprintable and can possibly track you. Even on desktop they’re behind. Chromium based is much more optimized and secure.

I saw on GrapheneOS forum Vanadium is the better option, with Brave a second alternative. Also tor browser on android is not officially from the tor project and suffers from firefox android implementation. Browsers are extremely complex, they’re made to run external code in the wild securely. Quite an endeavor.