1. Go to about:config, then accept the risk and continue
2. Type in browser.tabs.groups.enable
3. Double click the bar so that it is set to true
4. Close firefox and reopen

@ Mark

Thanks. It worked, and I didn't need to restart, after I closed the config tab.

I have the same problem. Tried Mark's solution, but no luck. Still can't combine tabs.

Hi Charlie,

If the above information does not resolve your issue, you should consider creating a new thread with specific details about your issue.

Doing so will allow support forum volunteers to give you solutions that are more helpful to you. See the Mozilla Support rules and guidelines for more information about posting support requests.

Please, feel free to post the link to your thread on this thread for volunteers interested in assisting you.

Thank you.

I had the opposite problem, it WAS stacking tabs and this told me how to STOP it. Thanks