(Please start new threads for new issues — the thread you posted to was years old and about a different issue. I've moved this to a new thread.)

The most likely cause of this is just the document preferences window or citation dialog appearing behind another window. Those windows are part of the Zotero app, not your browser, so check for other Zotero windows.

If you can't figure it out, restart your browser and Zotero and provide a Debug ID for the first attempt to do this.