For me the search has a dropdown list now where I can select the search engine.

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For me the search has a dropdown list now where I can select the search engine.

Yes, the new one have a dropdown now.
It really just a matter of convenience for me, since in the old one it's just a single click away from adding a new search engine by clicking something like this.

Oh that's what you were looking for. Well, you can do that with one click like this: Go to a search engine you want to add, right click anywhere in the address bar, then you get that green +button to add it. Does that work for you?

That's still much less convenient than the previous interface. I'm forced to use my mouse on this new one, be it changing the search engine I want to use or adding a new one.
Anyway, I just found out it turned out to be the default from Firefox itself rather than a change from Librewolf. I just hope it doesn't affect Mullvad now, since I often experiment with different instances of Searx and 4get, and often change the search engine I'm using.
But, now I know where I have to ask next.
You might have a key for triggering the context menu on your keyboard. It's typically to the left of the right Ctrl-key. If you're on a laptop or small keyboard, you might need to hold Fn and then press another key down there.
So, if you press F6 or Ctrl+L, that should focus the address bar, and then you can press the Menu-key and then the up-arrow to get to that option.
settings>search
enable show search suggestions
?
Unfortunately, doesn't do anything.