Though it was made before inactive tabs grouping, so if this feature will get implemented (and I hope it will) they would probably use the same UI elements for custom groups as used for inactive tabs group.
Your idea has quickly received a great deal of votes (kudos) and support here in the Mozilla Connect community, so we are upgrading the status to โTrending idea.โ This means itโs now one step closer to reaching our internal teams for reviewโlearn more about The Idea Journey.
Please keep the conversation going and stay tuned for updates ๐