Works for PDF, jpg/png files not recognised by ff because of some misdescription of the link in the web site.
DOES NOT OPEN csv, doc* or xls* files- the dialog opens, server sent MIME and MIME from file extension both just open FF open with external editor dialog! Text, html , web open gobbledegook.
Marked down for wasting half a day figuring this out trying to figure out the teeny "other" dropdown menu which shows the beginning of lines (in the empty dialog spanning 2 monitors+) then snaps shut when you even try to screenshot it (hint-zoom page to max shows more of the lines width). If your addon doesn't do something, TELL US. If it does, instructions? or better still can dev set it up properly please (cf docs online viewer addon which DOES preview office docs in the browser when clicked.)
Dialog really does disappear if you tick "do this automatically" box. To restore dialog, Tools>open in browser>prefs, click "default" for the filetype.
Click "open in firefox" option to access "open with external app" dialog or open html link in ff.
Dialog expands wider than 3 monitors. notified dev, not fixed after a year. 6/6/2020.