The paradox being that every thing you customize about your browser config becomes another thing that can potentially be fingerprinted and makes you stand out as one of the 1% who has ever looked in about:config.

That's a common thought, but it depends what you touch. I have hundreds of user.js customizations related to local browser behaviour (e.g. null out a lot of upstream URLs, I caught FF making DNS queries to services I disabled) - https://coveryourtracks.eff.org/ reports I have extremely strong anti-fingerprinting. The "failures" are not related to Firefox.

Reading the details of the results, my unique values as reported come from factors which are hard to address; I have an Arch Linux user-agent (small population) and I have Linux fonts installed (we'll fail the span-font test easily compared to Windows or macOS) are the two huge outliers. These two are my heavily identified traits, the rest are a wash or normality ("1 in 3").

The fonts one is funny - the span-font metric for my system is 16.82 of 115876.67 kinda showing just how using Fonts you can pick a Linux user out of the results with ease. I have "the usual" font packages installed, nothing too fancy just enough to see CJK / UTF-8 around the web like everyone else. (for completeness I do remap 2 or 3 esoteric fdnts on my side due to a site using them).

Side note: I have webgl disabled in user.js; the site reports I'm 1 of 85 statistically, this being the third largest and only outlier in normality.