Comment 7 for bug 1196339

Addition:

I already have the new firefox 29.0 (Build identifier: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:29.0) Gecko/20100101 Firefox/29.0) installed. I also used strace to find out wich locations firefox 29.0 is searching for the folder handler (I only listed those files existing on my system in firefox's search order):

/home/wreimer/.local/share/applications/mimeapps.list
/usr/share/ubuntustudio/applications/defaults.list
/usr/share/xfce4/applications/defaults.list
/usr/share/applications/mimeinfo.cache
/usr/share/applications/defaults.list
/home/wreimer/.local/share//mime/mime.cache
/usr/share//mime/mime.cache

Applying "grep inode/directory" to these files yields:

/usr/share/ubuntustudio/applications/defaults.list:inode/directory=nautilus.desktop
/usr/share/applications/mimeinfo.cache:inode/directory=audacious.desktop;Thunar-folder-handler.desktop;
/usr/share/applications/defaults.list:inode/directory=nautilus-folder-handler.desktop
Binary file /usr/share//mime/mime.cache matches

So the files /usr/share/ubuntustudio/applications/defaults.list (from package ubuntustudio-default-settings) and /usr/share/applications/defaults.list (from package desktop-file-utils) specify Nautilus as default folder handler. However, in Ubuntu Studio Nautilus is not installed by default so firefox uses the first entry in /usr/share/applications/mimeinfo.cache which is Audacious.

The attached patch will resolve this issue on Ubuntu Studio 14.04 (trusty) globally (for all users). IMHO the bug is NOT in audacious or firefox. In my case the bug is in the package ubuntustudio-default-settings (the patch can be used to fix the package).