Prevent named actions from displaying a 'resource://' URL in Firefox

After #3557 was merged there is support for named actions. One small visual issue with that PR is that in the *Firefox addon*, when hovering over a named action link, a `resource://` URL will be displayed. This is fixed by this PR.

An example of a file containing named actions: http://homepages.cwi.nl/~frb/PC13slides.pdf.