Hi Magnus, I have been using mix'n'match for a few days and this is a fantastic tool. I just wanted to let you know of a bug I encountered a couple times in both Firefox and Chrome.
In a nutshell, the page freezes after clicking 'not on wikidata'. It doesn't create the new item, yet the action 'not on wikidata' gets recorded in 'recent changes'. It could be deceptive to users who assume the item was successfully created by the program.
This bug is not super common (for me it occurred maybe once for every hundred uses of the tool) and the only reason I noticed it is because sometimes I want to add more statements manually and the item isn't listed on my 'contributions' page. One more thing I'd like to mention is that I don't have any extensions in Chrome that could interfere with the program. Anyways, thanks a lot for creating the tool, and I hope my feedback is useful.