Go to about:config in the address bar and make sure browser.tabs.insertRelatedAfterCurrent is default true.

Thanks for watching and replying.

It is set as it should, the default. And the old behavior is partly back. Could this be theme related? That's the only thing I changed as far as I remember.

Now the middle click opens a new tab (right position of the active one), but my standard search machine is shown searching for the content in my clipboard (strg+c)?

There are these middleclick prefs that you can look at (both true for me).

• browser.tabs.opentabfor.middleclick true
• browser.tabs.searchclipboardfor.middleclick true

Thank you very much. Setting the last one

browser.tabs.searchclipboardfor.middleclick false

seems to do the trick and the old behaviour (at least for me?!) seems to be back.

In a new profile without any modifications, browser.tabs.searchclipboardfor.middleclick doesn't make any difference to where new tabs are created when middle-clicking the new tab button.

See also:

• https://searchfox.org/mozilla-release/source/browser/app/profile/firefox.js#849

// Does middleclick paste of clipboard to new tab button
#ifdef UNIX_BUT_NOT_MAC
pref("browser.tabs.searchclipboardfor.middleclick", true);
#else
pref("browser.tabs.searchclipboardfor.middleclick", false);
#endif

• 1418462 - Middle clicking on new tab should search or open what's in the selection clipboard on linux [115]

See also:

• 1841763 - Firefox 115 broke "New tab next to current tab" middle mouse click functionality of the new tab button