Add hint on FF addon. Fixes #201.
Fixed grunt jshint:gruntfile.
Placed error back on line + added / moved new-lines.
Changed text according to @sun.
Dreditor 1.2.9
Reformatted + rephrased error string.
Merge pull request #212 from clemens-tolboom/feature/hint-ff-addon

Add hint on FF addon. Fixes #201.