Kubuntu 12.04 Firefox missing file associations to open downloads

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http://ubuntuforums.org – I just installed Kubuntu 12.04 beta 1. I used the included menu item to install Firefox. No problems with the installation. But when I download an item (e.g., a JPG image) with Firefox and then choose to either open it or open the containing folder, I get a dialog titled "launch application" and I have to "choose an application". This happens for all application types. What is the best way to resolve this? EDIT: in Firefox preferences > Applications, the only content types shown are irc, ircs, mailto, podcast, tar file, webca and webfeed. Normally, there would be man (Hardware)