I know this is an odd question to ask here - some might say its not the place to raise it but in the unlikely event that I am right, it could effect others who depend on this group. Basically a somewhat popular firefox plugin might have some undocumented and possibly unwanted features.
I use Firefox 47.0.1 on a Windows 7 environment with a couple of plugins as part of my WAMP environment. I use console.log generously, and noticed today (possibly since yesterday) that my console included the word "Testing" in the output. It did not attribute a source to it (typically on the right side of the console it will give me the script and a line number - it was just blank). The word "Testing" would appear at the same position, after a couple of my own functions had called. My entire javascript code base is near 4000 lines - I could not recall putting such words in, but as a precautionary measure, I copied it to a cygwin/*nix command line and searched grep -i "testing" through my scripts and did not find the word. I tried other creative ways to help me identify where it might be kicking in but failed.
I use Firefox profiles - one specifically for development that has the disk and memory cache disabled. I uninstalled the plugins, deleted the profile, then recreated the profile, re-ran my code.
My code run's as before with the exception that the "Testing" no longer appears within the console output.
Am I being paranoid? I cannot think of where the word "Testing" would come from. Nobody else has access to this laptop (trucrypted hard disk with 20character password, antivirus, not shared with anyone).
You can use the browser toolbox to search through all loaded javascript code at runtime.
In case you have multi-process enabled you will also need to use the content process toolbox to search through javascript loaded into the child process.