Yeesh, using Firefox on Kubuntu sucks almost as much as not using Firefox on Kubuntu. It's a nice browser, but I download a file, then in the download list, I try to open it, and it says "Oh, what would I open a PDF file with?" and you have to walk through finding acoread, and saying "Use this, and please remember for next time". Then you download another file type - maybe a Word doc - and you get to repeat the whole process for that file type, and so on till you could scream! Then Firefox would probably ask "And what should I use to open a .scream file?"
It's not Firefox's fault, really - there seems to be no uniform way to do this across desktop environments - witness the havoc that ensues when otherwise intelligent people start discussing this issue.
Anyway, buried waaaay down on that page is a suggestion to use xdg-open - essentially a tool that uses your desktop manager's native preferences to decide how to open a file. It's not a perfect solution, but by golly it works! For a minimal config, just use xdg-open to open the containing folder, then rely on your native file manager to provide app preferences for opening the contained files.