Letterboxing background colour is no longer used

Upstream bug 1941635, which seems to be a confidential issue, moved

background: var(--tabpanel-background-color);

from #tabbrowser-tabpanels to .browserContainer, so our letterboxing override for #tabbrowser-tabpanels no longer has the intended effect.

It might make sense to instead adjust --tabpanel-background-color to be transparent instead of overriding the background property.