emerge -pvc x11-libs/gtk+:2 Calculating dependencies... done! x11-libs/gtk+-2.24.32-r1 pulled in by: mail-client/thunderbird-78.7.0 requires >=x11-libs/gtk+-2.18:2
Mozilla products will lose their gtk+:2 dep once NPAPI code cleanup is done, but unsure if this will ever happen for 78.x versions.
Apparently fixed in commit 58c7ce08ee50d512f49f9162b63dec7be49fc497