Curiously, that was FF 123.0b2, on macOS 13.1. Pretty close.

These things are almost always bugs in JavaScript running on the site, IME.

If I inspect a tt, it's setting a background of slate (black), and a foreground of "inherit", which is black. This is just messed up CSS, and the inspector seems to indicate that the JS on the page is crashing, and I see there's a sun/moon hieroglyph in the corner, i.e., there's JS for controlling light/dark mode.

It seems like the code responsible for the light/dark mode must run one of these two things:

  document.documentElement.setAttribute('data-bs-theme', 'dark')
  document.documentElement.setAttribute('data-bs-theme', 'light')

If neither happens, the page is left with the default styling, which is a bizarre mix of light/dark mode and contains the issues I've screenshotted.

Neither happens, because slightly prior to that, the JS makes a call to a fallible function, and then subsequently fails to handle the error case. The uncaught exception kills it before it can call one of the two above lines, so we're left with a page with broken styling.

It's a JavaScript bug.