Since Firefox isn't a GTK application, it won't work with the Editable Menu Accelerators option. Also, the Firefox help page on keyboard shortcuts says, "Firefox does not provide any method of customizing keyboard shortcuts." (It also shows you can use either Ctrl+K or Ctrl+J on Linux versions, but not Ctrl+E.)
However, there is a way to do it! There is an extension for Firefox (and other XULRunner applications) called Keyconfig (install). According to the MozillaZine post on the subject:
keyconfig adds the ability to create new or modify existing shortcuts defined by a element, but only changing those which itself call a function (those with a command or oncommand attribute) has an effect (all others fulfill only cosmetic purposes it seems and are grayed out).
The configuration screen can be accessed via Tools > Keyconfig (in most Applications) or Ctrl+Shift+F12 (Command+Shift+F12 on MacOS) from the main window.
Don't think that changing a will always remove the original shortcuts. For Example: You can change the shortcut for Copy in the Bookmarks Manager but the original shortcut will still work (in addition to the new shortcut).