Sometimes I want to open e.g. an email link in a specific tab group rather than just at the top of the tabs list. I haven't found an addon that does this either.
Right now, there are two potential workarounds that might work for you: 1. Right-click a tab that's already in the tab group, and click the New Tab Below option. That will open a new tab within the group, right below the tab you clicked. 2. A more radical approach that might or might not work for you depending to your case: if you go to about:config and change browser.tabs.insertAfterCurrent to true, any new tab you open will open next to the current tab (regardless of whether you use Ctrl+T, "+" tab button). We're planning to make it easier to add a new tab within a group that doesn't involve these workarounds, but the horizon for this is about ~4-6 months.