Reproducible for me in Ubuntu 24.04, Firefox 150.0 from Mozilla’s PPA, with a small difference: new tab button does seem to work, which is useful, since without it wouldn’t be possible to open about:config the workaround of toggling widget.wayland.fractional-scale.enabled.

A small addition: Firefox 150.0 (from Mozilla’s PPA) is similarly affected for me, so this is indeed an upstream one.

All right, thanks for looking into this — whatever the outcome.