but it should be "/usr/bin/thunderbird %u" This upper case/lower case issue might happen multiple times. Reproducible: Always
Could you please check if the Exec line in thunderbird.desktop has the correct option?
gah - that's the problem, right : $ grep -i exec /usr/portage/mail-client/thunderbird/files/icon/thunderbird.desktop /usr/share/applications/thunderbird.desktop /usr/portage/mail-client/thunderbird/files/icon/thunderbird.desktop:Exec=/usr/bin/thunderbird %U /usr/share/applications/thunderbird.desktop:Exec=/usr/bin/thunderbird %U