Five stars for legacy version 1.18 – tested OK with Waterfox Classic with the URLs below, which were problematic with alternative extensions:

https://github.com/progit/progit2/releases/download/2.1.109/progit.pdf

https://www.st.com/resource/en/datasheet/stusb4500.pdf

https://ec.europa.eu/newsroom/dae/document.cfm?doc_id=62684

I assume that version 2.9 for Firefox Quantum is equally good.

My previous review:

https://web.archive.org/web/20191209154542/https://addons.mozilla.org/en-GB/firefox/addon/open-in-browser/reviews/902033/

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.

A really nice extenstion :)

I give it 5 ***** as soon as this can be fixed: https://github.com/dimyme/djvujs/wiki/associate-.djvu-filetype

Have been using this for ages. Firefox just doesn't work right without this addon :-)

Useful for dealing with resources where the server sends an incorrect mime-type.

Recently, for some reason, when I tried opening images in a new tab a Save prompt was displayed instead. This extension fixed the problem.

An essential tool to avoid filling your Downloads folder with temporary files.

I feel I have to repeat it:pretty useful and well made.
More I use it more I appreciate it.Thankyou!

simply perfect

This is one of my favorite extensions for Firefox, but it's having some problems with Firefox 57.0. It seems to behave badly on some pages that redirect you at load time (unfortunately, my best example so far is a company-internal page which I can't share). I'm also getting prompted twice now when downloading things -- once to present the traditional "Open with..."/"Open in browser as"/"Save File" dialog, and then again with another "Open with..." / "Save File" dialog.