Comment 3 for bug 950365

I had same problem here and solved with this steps:

1) stop firefox
2) rm ~/.mozilla/firefox//mimeTypes.rdf
3) start firefox
4) download a file
5) open it from the downloads menu and choose the application. Browse to /usr/bin/xdg-open. then check to remember your choice

source: http://matthieu.yiptong.ca/2012/04/12/fix-file-open-and-open-containing-folder-on-kdekubuntu-12-04/