How do I hide one of those entries? only the correct one is in my bookmarks.

Are you sure about that? The star over each icon indicates that both of them are bookmarked.
So either this is a bug or there is a second bookmark hiding somewhere. If you go to Bookmarks > Manage Bookmarks and search for "qb", what appears there?
- so there was no other bookmark for this, but it made me think again what could have gone wrong... so i visited this wrong url -> did not connect of course, i put it in bookmarks and it autofilled the quick term (don't know the english word for that) i undid that, saved the bookmark et voilà! thank you for the inspiration

To delete old suggestions based on previously visited urls you should be able to press the down arrow to select it and then shift + backspace to delete its memory of that suggestion. Sometimes I have to do it multiple times if multiple similar urls that don't exist anymore are still being suggested.
This is assuming the incorrect one isn't also bookmarked or being suggested for some other reason besides being visited in the past.
- press the down arrow to select it and then shift + backspace to delete its memory of that suggestion
  Bless you, kind lemming.
- that's good to know, I'll try this. for some reason firefox always tries to recommend facebook as the first thing when I start typing something similar, even though I left it 5+ years ago

I've got the exact same problem lol
- Try pressing down to select it and then shift + backspace to delete the memory of it. I have to do this a lot to clean up old self hosted urls that it keeps suggesting.
  
  Same but I just press delete instead of shift + backspace