Firefox's handling of the "content-disposition: attachment" header can be quite confusing, to put it mildly. There's a bug report about this that's nine (!) years old (https://bugzilla.mozilla.org/show_bug.cgi?id=453455) and that's still open and unassigned.

The Inline Disposition 2 extension used to fix this quite nicely, but with Firefox Quantum that's no longer an option. Since upgrading to Firefox 57 I've tried a couple of WebExtensions to get rid of those unnerving "Always do this from now on (haha, just kidding, still enjoying your Chinese water torture?)" messages, but none of those really worked for me, including this one.

So what I wrote earlier about the "Display inline" extension is also mostly true for "Bypass Forced Download": Nicely done with a simple interface and very well-documented, but it doesn't seem to work everywhere. For example the .torrent files on http://hdreactor.info/ are still delivered as attachments with this extension active. Lukas, maybe you want to have look at this issue? I'd file a bug report but you don't seem to have a github or similar site.

EDIT: I've now replaced this extension with Header Editor (https://addons.mozilla.org/en-US/firefox/addon/header-editor/), a more generalized extension where I could quite easily define a rule to ignore those those pesky Content-Disposition: attachment headers on any website, including the one mentioned above.