The thing just much more convenient to use. Unless there's some security/privacy related thingy that forces the change.
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.
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.