When I say search for an already open tab I mean it could be a tab which is already opened in the current tab group or a different tab group which is hidden. I remember seeing a video on this a long time ago - Tab Candy, maybe? But I think that was an add-on..
In the Tab Group view, you should be able to just start typing to search for tabs. If the title of any tab, in any group, matches the search string, that tab will be highlighted.
By default you can start typing in the location bar and Firefox will search open tabs, bookmarks and history. Notice that in the screenshot below, I'm performing a search from a tab group that only has a single blank tab. Firefox finds my open Stack Exchange tab even though it's in another tab group:
You can take this a step further, and search only open tabs by using the
Edit: I've updated this answer to include a number of examples which may be useful to the OP or other passersby. If the Location Bar autocompletion isn't working as expected, it may be due to a configuration issue. The following links provide some information about relevant