@cstross another endorsement for Vivaldi, but I understand you are invested in Firefox and can't easily switch.
Safari is the most energy efficient browser on macOS, thus a good fit for your Neo. Unfortunately, in line with Apple's collapsing software quality, it is rife with web standards compliance bugs. Apple is also all-in on advertising, and the corrupting influence is seeping through the company. For instance, in a "one rule for thee, but not for me", they exempt themselves from the advertising ID opt-in they impose on others because "we're not a third-party", the EU is taking a dim view on this, but according to a classmate who works at the EC's antitrust division, Apple is the most arrogant and entitled of the FAANGs.
I myself switched to Linux, and use Vivaldi fully locked down (cookies disabled by default unless allowlisted, uBlock Origin, Privacy Badger, Global Privacy Contol turned on), and a vanilla Firefox for the sites that won't render on Vivaldi (typically Javascript-heavy ones that break if cookies are disabled, grrr), but I have a button that nukes all cookies and state in Firefox.