This market resolves according to the date when the first general public stable channel release of either upstream Chromium or Mozilla Firefox is officially published that completely drops the ability to run natively under the X Window System on Linux.

This means that standard release builds can no longer run as an X11 client natively (e.g., removing the X11 backend.

I'm referring to a loose idea of "standard, upstream, stable channel, release build" in that, e.g. distro packages patching in support don't count, beta versions don't count, etc.

"Locked behind a non-default runtime option" doesn't count as not supporting X11, "locked behind a non-default compile-time option" counts as not supporting X11.