You might try going to about:config in the address bar and see if browser.tabs.groups.enabled is set to false. If not, change the value to false.

That should make the feature go away.

Update: corrected my response.

The about:config workaround works for me. I am sure it is a nice feature for some... But for other users like me, and probably older either, it is a true pain in the neck when it groups automatically things, you don't notice it or delete the group by error, and end up searching for the lost tab(s) re-opening it again. Especially boring and site-breaking when it is a secured site with a specific 1-time security (ex, some gov sites), or uses various containers, etc... In short, not friendly when not wanted/controlled (without going under the hood, but thanks to MarkRH). The option to turn on/off should be added in the settings, asap, under General / Tabs.

Thanks, I can't believe this is not in settings. I sometimes rearrange the order of my tabs and it automatically grouped them and can't believe someone thought this was agood idea

The about:config doesn't work for me. I changed it from "true" to "false", restarted my computer, checked and verified that the browser.tabs.groups.enabled is still on "false" but the tabs grouping continues! Often when I work, for example when I do simulation experiments, I need to have both the instructions page and the simulation page open side by side for the experiment to go smoothly instead of toggling between tabs to see which one is which and sometimes loosing the tab I want completely and have to start over. I truly hate this tab tab grouping thing! Firefox has been my default browser for several years now; but that's about to change because this tab grouping is hampering my work.

The about:config doesn't work for me. I changed it from "true" to "false", restarted my computer, checked and verified that the browser.tabs.groups.enabled is still on "false" but the tabs grouping continues! Often when I work, for example when I do simulation experiments, I need to have both the instructions page and the simulation page open side by side for the experiment to go smoothly instead of toggling between tabs to see which one is which and sometimes loosing the tab I want completely and have to start over. I truly hate this tab tab grouping thing! Firefox has been my default browser for several years now; but that's about to change because this tab grouping is hampering my work.

After you set browser.tabs.groups.enabled to false, you then need to ungroup tabs that have been grouped. Right-click on the colored box to the left of the group and then click "Ungroup tabs".

**Alternately**

For you or anyone reading, Firefox recently added a way to reduce accidental grouping. To do this, increase the delay before Firefox suggests group a tab you're dragging with the one it's positioned over. Here's how:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.

(2) In the search box in the page, type or paste browser.tabs.dragDrop.createGroup.delayMS and pause while the list is filtered

(3) Double-click the preference (or click the pencil button) to display an editing field, and change the value to 1500 (for 1.5 seconds), then press Enter or click the blue check mark button to save the change.

That should eliminate most accidents. You can still create groups by multi-selecting tabs and then right-clicking any of them to access tab group actions.