I updated to Firefox 40 today, and I see a neat new message in my Firebug console:

Found hi-entropy localStorage: 561.0263282209031  bits http://localhost:8080/my_app_path itemName

...where itemName is the name of a particular item I've stuck in localStorage.

The referenced line number is always unhelpful: the last one of the main HTML document (it is a single-page app).

Why does this happen? If you'd like an example of my "hi-entropy localStorage", here are the data in question:

Object {
 id: "c9796c88-8d22-4d33-9d13-dcfdf4bc879a",
 userId: 348,
 userName: "admin"
}

I updated to Firefox 40 today, and I see a neat new message in my Firebug console:

Found hi-entropy localStorage: 561.0263282209031  bits http://localhost:8080/my_app_path itemName

...where itemName is the name of a particular item I've stuck in localStorage.

The referenced line number is always unhelpful: the last one of the main HTML document (it is a single-page app).

Why does this happen? If you'd like an example of my "hi-entropy localStorage", here are the data in question:

Object {
 id: "c9796c88-8d22-4d33-9d13-dcfdf4bc879a",
 userId: 348,
 userName: "admin"
}

• javascript
• firefox
• local-storage

• 3 Do you have Privacy Badger installed by any chance? – approxiblue Commented Aug 13, 2015 at 1:48
• Hmmm, yes. In fact, that was my first thought based on the message, so I disabled it for the site in question. I guess it's time to disable it entirely and see if that fixes the problem. If it does, then Privacy Badger certainly isn't playing very nice--it's not disabling for a site when asked, and it's also not identifying itself in the message. Time to restart the browser, I'll be back in a few minutes. – Lambart Commented Aug 13, 2015 at 16:58
• So I learned two things today: a) Privacy Badger doesn't respect its settings properly, and b) There is at least one Firefox addon you can (apparently) disable without restarting the browser--because that seemed to work. – Lambart Commented Aug 13, 2015 at 17:11
• Also annoying: Privacy Badger throws cryptic messages at the console without identifying itself as their source. Thanks @user880772 -- if you post an answer I'll give you credit. – Lambart Commented Aug 13, 2015 at 17:13

2 Answers 2

Your browser has the Privacy Badger plugin (1.0), which can detect some types of super-cookies and browser fingerprinting. It identified your local storage item as a false positive and produced those cryptic logs.

A high-entropy string can be vaguely defined as plicated, hard to guess/repeat, or likely to contain meaningful information. If there's such a string in your local storage (in your example, the item id), it's possible that advertisers put it there to uniquely identify you. Privacy Badger has rough methods to estimate a string's entropy, which the developers discuss here.

You should check out the paper The Web never forgets: Persistent tracking mechanisms in the wild, particularly the section on cookie-syncing:

Cookie synchronization or cookie syncing is the practice of tracker domains passing pseudonymous IDs associated with a given user, typically stored in cookies, amongst each other.

I guess is a stranded value. I disabled a script from zopim chat and this started to show. looking for what entropy means I found this explanation "(in data transmission and information theory) a measure of the loss of information in a transmitted signal or message. " which make sense.

You can see what is in Local Storage by opening Developer tools (Ctrl+Shift+S) and enable Local Storage panel by pressing Toolbox option in the right side of the menu bar. To delete the value in question, just follow the steps from here How to view/delete local storage in Firefox?