In light of the recently-added tab groups feature, one feature which I think would be really welcome would be a keyboard shortcut (similar to Ctrl + T) which opens a new tab directly to the right of the current tab, in the same group as the currently-active tab.
Before I switched from Chrome to Firefox, I was an avid user of the "New Adjacent Tab" extension, which implemented this behaviour. There is a similar add-on for Firefox ("Open Adjacent New Tab") but it does not perfectly replicate the behaviour of the Chrome extension when the currently-selected tab is the rightmost tab in its group. I think this would be a helpful keyboard shortcut to have in Firefox for easier tab management, particularly for creating and managing tabs within groups.