Currently, there are shortcuts to move up or down a tab group (CMD + Shift + up / down). I think it would be quite useful to have an option to select tab groups based on hotkey (i.e. CMD + Shift + 1 to move to the first tab group). I really like the current way that tab groups work, and I switch between them frequently enough that I think this could be really useful.
I can see a user who frequently switches between tab groups able to do so without paging through each of them one-at-a-time. There could also be an option for CMD + 1-9 to switch between tab groups rather than bookmarks or tabs. I personally don't use my bookmarks enough, and often have way too many tabs to make use of that keybinding, so it might be a good candidate for this feature.
