I've compiled my firefox without wayland support but it still runs as a Wayland client on my Plasma wayland session. If that USE flag cannot disable Wayland then there is no point of having it... It worked fine on 68.7.0-r1 Reproducible: Always
See what's depending on USE=wayland. In case your system is not wayland-enabled, this will pull in required wayland stuff. But firefox has no hard build time dependencies. Also, USE=wayland will control default start mode.
Oh yes, default start mode is now working for me again. I think my system was somehow messed (and default start mode was not working) up but reboot fixed it. So good to close as invalid. Sorry, should have rebooted before reporting.