I have a WIP patch that switches back to 16/32/16/32 when an addon gets pinned to the toolbar. Also, you can work around the issue by using SVG. https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/icons#svg
Bug 1811128 Comment 14 Edit History
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You can work around the issue by using SVG. https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/icons#svg