How can I change the settings for OS X Lion so that the combination of Shift+End results in the selection of text from the current cursor position to the end of the current line? Specifically, I would like to be able to do this in Firefox, but if there is a general method that works for most/all applications, that would be ideal.
There is a default key binding to perform this function: shift + ⌃ (control) + →.
If this won't serve, you may be interesting in Jacob Rus: Customizing the Cocoa Text System which provides instructions for creating your own keybindings dictionary.
Basically you could create ~/Library/KeyBindings/ and save a property list like this as DefaultKeyBinding.dict:
The custom keybindings are currently not supported by Firefox though.