I guess the market share is tiny, maybe thousands of users not millions of users.
I've found Pale Moon handy over the years; I used to use it for years as an early 64-bit browser. I guess I can't worry if xenforo isn't compatible with 0.003% - 0.013% of the worldwide market, but it seems like only a small appearance glitch with the popups/overlays. (I'm typing this message in Pale Moon to test, and all the major functionality except for the overlays, thread previews, and lightbox seems to work fine.)
Would be curious for any info on what the feature is here that pale moon doesn't support for these overlays.
Or if there is any easy tweak for backward compatibility.