Instead of Quick Filter, right-click the folder, Search Messages, and create a filter like the one in the picture. Save as Search Folder if you do this often.

Yes, this is what I was looking for.

However, I use unified folders, because I use two mailboxes. The search folder now is a subfolder of one of my mailboxes. Is it possible to display this as a separate folder? Just like the inbox of unified folders.

I think you could create the Saved Search from File/New/Saved Search... according to the method described here, and include the Inboxes from both mailboxes ('Select the folders to search').

That is indeed what I did. However, I have to choose one of my inboxes as the parent folder of this search folder. I would like it to be on top, just like with unified folder view.

I have found another working solution. I made all important folder, so also the search folder, a favorite folder and switched to 'Favorite Folders Compact View'.

Thanks a lot!

-edit-

I do encounter another problem. When I restart Thunderbird, the search folder is not on the list when it is set to 'Favorite Folders Compact View'. When I switch to another view and back, it returns. When it is set to 'Favorite Folders' it is on the list on startup.

I'd have suggested Saved Searches too, but I rarely use Unified Folders.

Another useful tool is the Mail Views widget. Open your mail toolbar for customization and add the Mail Views tool. It comes with a couple of built-in presets, but you can add your own custom views. ISTR it uses settings very much like the regular filter dialogue so it should be able to do the same as the Saved Search.

Ah, that works great! Zenos and sfhowes, thanks for your time and support

I would pick both as problem solvers, but Zenos' reaction will help other people better, I think.