Apps not starting

hi.
in my mac osx 10.6.7 some apps do not start (firefox, emacs, texshop, etc.). they just bump over the dock for a while and then stop without showing anything.
any idea? is it a permissions issue?

Yes it could well be a permissions issue. So go ahead and repair them via DU.
If not that then try and get the .plist for said Apps ~/Library/Preferences/ and drag .plist to the desktop and now restart the App. It will create a new .plist and if it works trash the .plist you have on the desktop. If it doesnt work, just replace the .plist from the desktop back too the Preferences Folder.

So after all that if it still doesnt work, post back and see where we can go from there.

Cheers

Already tried with DU. Just tried the .plist method and the result is always the same.
I have problems with sudo. it says "cant stat /etc/sudoers". does it influence my problem?