Question: I have a pretty weird problem. I have an iMac and have just had my old HDD replaced with an SSD. The old HDD is now in my optical drive's slot.

I have a clean installation of El Captain on my SSD. Unfortunately, the guys who swapped the drives for me installed El Captain in Polish. Now I changed the OS language to English as I'm more used to it, and this is what my Finder does.

[![enter image description here](https://i.stack.imgur.com/sPxie.gif)](https://i.stack.imgur.com/sPxie.gif)

I still have an old OS installed on the other disk, which is not formatted yet. I thought that it might be by some miracle conflicting with the new installation, but Finder does this even after unmounting the old HDD.

I have absolutely no idea what to do with this, and it gets very annoying.

Has anyone had this problem before? Uncle Google brings no answers on this one :|

Answer: 
Well, seems that Firefox will not fix this [issue](https://bugzilla.mozilla.org/show_bug.cgi?id=67940) but I fixed it in my way removing the `application/octet-stream=org.gnome.gedit.desktop` from my `~/.config/mimeapps.list` and using the add-on [OpenDownload²](https://addons.mozilla.org/en-US/firefox/addon/opendownload-10902/?src=userprofile) to provide me the `Run` option.

If the extension of the file being downloaded is correctly configured in the `mimeapps.list`, firefox will open **(run)** it like a charm.
@Bob answered part 2 of the question: how to control which app opens the file.

The answer to part 1 is: Firefox downloads all files of type application/octet-stream **by design**. According to the HTTP standard, [a compliant browser MUST NOT second-guess the server if a content-type is specified](http://forums.mozillazine.org/viewtopic.php?f=38&t=1303555#p6725425). Since octet-stream files contain arbitrary data, the browser should download them.

Most browsers ignore the standard in favor of user friendliness. Firefox does not. As stated in @Geyslan's answer, you would need to use a 3rd party extension to circumvent this design choice.