Can you double-check your settings here:

orange Firefox button > Options > Tabs

or

Tools menu > Options > Tabs

"Open new windows in a new tab instead" - unchecked
"Always show the tab bar" - unchecked
"Show tab previews in the Windows taskbar" - unchecked

If new windows are still being diverted to new tabs, is this consistent or only on certain pages?

All that is ever unchecked by me. But it happens aal the time on all webpages. What is that?

Henk.

It could be related to one of your add-ons. Do you have any that seem to be related to tabs? You can check here:

orange Firefox button > Add-ons > Extensions

or

Tools menu > Add-ons > Extensions

If there's nothing obvious, you could try Firefox's Safe Mode as a diagnostic.

First, I recommend backing up your Firefox settings in case something goes wrong. See Backing up your information. (You can copy your entire Firefox profile folder somewhere outside of the Mozilla folder.)

Next, try starting Firefox in Firefox Safe Mode. Be careful not to "reset" anything permanently if you didn't back up.

Does that bypass the problem? If so, you can disable all add-ons and then gradually re-enable the most important ones until you figure out which one is causing the problem.

What orange button do you mean: my orange button (that in the left upper corner?) gives only move, change format, minimalize, mamalize and close.

The orange Firefox button is the new consolidated menu that appears if you get rid of the classic menu bar. If you don't see an orange button that says Firefox, then use the Tools menu.

(It's very hard to provide help here when the UI can have so many different appearances!)