Take a long-ish article - Obama, https://en.wikipedia.org/wiki/Death_metal, you choose.
Try and re-use a reference.
Here's what I observed -albeit sporadically:
- it takes a lot for the Cite menu to even come up;
- after searching for a reference to re-use, click on it: it isn't added to the article, and you can't go past the search results;
- clicking on Cancel also doesn't seem to accomplish any effect.
After point 2 in a different attempt I tried to click on a random link on the page - that worked, in that the dialog disappeared and the reference was where I meant to place it.
Another user says they get a warning from Firefox about a script being slow/unresponsive while trying to add a citation.