The "Sidebar Revamp", Mozilla rolled out a new design for the sidebar that changes its default behavior. The new sidebar is part of an ongoing effort to modernize Firefox's user interface.

How to Restore the Old Behavior

There is a workaround to get the old behavior back, but it involves going into Firefox's advanced settings, which you should do with caution.

1. Go to about:config

   Open a new tab in Firefox.

   In the address bar, type about:config and press Enter.

   A warning page will appear. Click "Accept the Risk and Continue."

2. Change the Sidebar Setting

   In the search bar at the top of the about:config page, type:
   sidebar.revamp.enabled

   You should see a single preference appear. The value is likely set to true.

   Click the "Toggle" button (a small arrow) to the right of the preference to change its value to false.

3. Restart Firefox

   Close and re-open Firefox.

This should disable the new sidebar interface and revert it to the old, classic design, which should restore the original click behavior you are used to. You should once again be able to left-click to open a link in the current tab and middle-click to open it in a new one.

Note: Be aware that this is a temporary workaround. As Firefox continues to evolve, this setting may eventually be removed, or the functionality may be integrated into the new sidebar in a different way. However, for now, it's the most reliable way to get your preferred behavior back.

or sidebar.revamp

That fixed it thanks a lot.