Add-ons are one of Firefox's strongest features. I'd say that the browser is offering the best platform for add-on developers right now, and users of the browser can select from thousands of extensions for the browser in the official store.
I uncovered some shady business practices in the past which did not really cause the echo that I had hoped for, which came as a surprise considering that hundreds of thousands of Firefox users are affected by that.
Some business buy established add-ons from their authors and add forms of monetization like ad injections, replacing of advertisements on web pages, cookie dropping or tools to spy on users, to the extensions.
Neither the change in ownership nor the code modifications are reported transparently to users so that many may end up with add-ons that they would have probably installed otherwise if they knew about the monetization features.
I'd like to provide you with two methods to find out if extensions or scripts manipulate web pages in a way that they have not been designed for initially.
The NoScript extension blocks all scripts that run on a site by default. This includes first party scripts, that is scripts that run from the domain you are connected to, but also third party scripts which get loaded from other domains or servers.
Manipulations are often loaded from third party web servers or domains which NoScript blocks from happening initially.
But you are also informed about that connection by the extension, so that you know that some extension or plugin that you have installed in the browser is making that connection.
It may sometimes be difficult to find out if an add-on is indeed responsible for that, or if the website makes the request instead. The easiest way to find out is to run Firefox without extensions (but NoScript) and connect to the website again. If you find that connections are no longer listed here, enable your add-ons one by one to find out which is responsible for that.
If you do not like to run NoScript because you believe that it is to complicated or reduces your browsing experience too much.
The web console displays all connections that the browser makes. It may take you a while to browse through them all, and a suggestion that I have in this regard is to enter http into the filter form to only display connection attempts.
To open the Web Console do the following:
The console opens up in a new window that is independent from the Firefox browser window. You can however attach it to the web browser if you prefer to work this way.
Note that a connection is not a surefire way of determining if an add-on injects ads or adds other contents such as cookies to the browser. It can also happen that these scripts only run on select sites, for instance Amazon or eBay so that you may not notice it during other connections that you make.
You can obviously also use network monitors to find out which connections are made by your browser. Wireshark is a popular tool for example that you can use for that purpose. You may need to spend some time getting used to the program however.
If you just want to check up on the add-ons and scripts installed in Firefox, then you can use the two methods mentioned above for that purpose. Especially NoScript is worth mentioning here as it not only detects connection attempts but also blocks them by default.
You can get a good fix on many extensions offered for Firefox by simply reading through the most recent comments posted by users of the browser. While that may not stop new code from trying to inject ads or other things into websites, it at the very least takes care of several popular add-ons that use this monetization method.
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