If you get in a situation where the feed app you've selected does not exist (or LS has "lost" it or you've selected "" on 10.3--see bug 352760), you get an error dialogue: *Application does not exist* FeedAppDoesNotExistExplanation That string is missing; it should be similar to BrowserDoesNotExistExplanation (I'm not sure I like "Application does not exist", but maybe it doesn't seem so sucky when there's explanation text.)
The BrowserDoesNotExist version includes "no longer exists", which works OK because on all supported OS versions a default browser would have existed, but (at least currently) with feeds, you can get a situation where no reader ever existed, so something more like this seems better: "FeedAppDoesNotExistExplanation" = "There is no feed reader selected. Please choose an application to handle feeds."; (In any case, we can revisit all these strings as part of bug 343941 before 1.1)
According to discussion via #camino, the above String is r=smorgan/cl and a=ardissone/ss. "FeedAppDoesNotExistExplanation" = "There is no feed reader selected. Please choose an application to handle feeds.";
Checked into trunk and MOZILLA_1_8_BRANCH.