This is reproducible, at least here:
I have a folder set expanded in the file pane (great feature to be able to do that, by the way). I double-click a saved html file to open it in Firefox, then close Firefox returning focus to DO, then expand another subfolder by clicking the chevron. The file I viewed then goes into filename edit.
It did it in 13.9, and I just reproduced it in the 13.9.3 beta.
See the capture:
Pretty sure that's a bug