The label not displayed in 112 was a bug. The add-ons in question show the label in 102 as well. It has been a design decision (not by me) to have action buttons display their labels in Thunderbird. I introduced the optional `default_label` manifest key to allow add-on developers to enforce icon-only mode. I am open to drop that and follow Firefox and not display labels at all.
Bug 1824536 Comment 3 Edit History
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The label not displayed in 112 was a bug. The add-ons in question show the label in 102 as well. When the browser_action API was first added to Thunderbird, it was decided to have action buttons display their labels to match Thunderbirds general toolbar button behaviour. I introduced the optional `default_label` manifest key to allow add-on developers to enforce icon-only mode. I am open to drop that and follow Firefox and not display labels at all.