FWIW this is basically how my Thunderbird (91) looks (with a different UI theme of course) and i don't think i've made any special customizations :-P. If anything Thunderbird uses less vertical space due to not having all the big icons.
Fortunately Thunderbird (and Firefox) uses Gtk theming for the most part and i've used a Motif-y theme for Gtk3, so it picks up most things.
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FWIW this is basically how my Thunderbird (91) looks (with a different UI theme of course) and i don't think i've made any special customizations :-P. If anything Thunderbird uses less vertical space due to not having all the big icons.
Fortunately Thunderbird (and Firefox) uses Gtk theming for the most part and i've used a Motif-y theme for Gtk3, so it picks up most things.