It's both Safe (trick to lift restrictions) and Unsafe (bypass/removes security measures).
From your link:
"You are probably wondering what the few lines of code do, and whether it is save or dangerous.
The first bits of code, those added to the config.js file, load one of Firefox's configuration files and remove information from the Signed_Types constant in that file. It so happens that this constant defines the types of add-ons and extensions that need to be signed for be installed."
"The method removes the add-on signing enforcement in Firefox. It is rather surprising that it is this easy, considering that one of the main arguments for enforcing add-on signing is security."
Is there any reason that you want to remove Addon Signing in Firefox?