This is an improvement over my comment on the tab group feedback megathread: https://connect.mozilla.org/t5/discussions/seeking-your-feedback-on-tab-groups/m-p/112329/highlight/true#M43414 (it was just about alphabetical sorting but I think we can go further)
Note: using Firefox on PC, I don't know about mobile experience
Currently, the Add Tab to Group sub-menu always sorts tab groups by recent usage / access. This is not enough and cause the same troubles as in YouTube when adding a video to collection: for older tab groups you need to read through and find it, and you're never sure if it really exists or you need to create it first. On YouTube you can at least Ctrl+F the collection name on PC, but here you can't search like that, only type and search the first letter of a group name (I will open another thread about that).
My proposal is to have 3 sort modes for the tab groups to find them more easily:
Most recent first (the existing one)
By alphabetical order (easiest to find any tab group by name)
Same order as tab pane (this reflects the actual order of tab groups in the tab pane - easiest for people who ordered their tab groups manually by theme, etc. and who know the order by heart - plus, it makes the action closer to drag and dropping a tab within the tab pane into another tab group with all tab groups folded - also see this suggestion to auto-fold tab groups)
Related: Add Tab to Group sub-menu: add search tab group by name
since a good search allows to find the tab group we want even if there is no good sorting
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