Browser Extension

A browser is the program you use to open websites — for example Chrome, Firefox, Safari, or Edge. A browser extension is a small add-on program that you install into this program afterward. Once installed, the browser can do something it couldn’t do before. Some extensions block ads on pages. Others remember passwords, translate text, or show you prices from other providers while you shop online. You usually install them with two clicks from a catalog run by the browser maker, and they then appear as a small icon next to the address bar.

Why a small icon carries a lot of power

An extension sits in a very sensitive spot. It runs alongside you as you browse the web, and it is often allowed to read and change the content of every page. That’s exactly why it can be useful: an ad blocker has to touch the page in order to remove ads. But the same permission also means the extension could, in theory, read what you type into your online banking.

That’s why extensions are a recurring topic in security news. It happens time and again that a popular, harmless extension gets sold by its developer. The new owner then pushes malicious code through an automatic update. Users don’t notice, because the icon doesn’t change. A rough rule of thumb: whenever you install something, ask yourself why a tool needs these particular permissions.

Economically, extensions are also a gateway into someone else’s market. Anyone without their own search engine or their own browser can still position themselves directly between the user and the website via an extension. That’s exactly what many AI providers are currently trying to do.

What parts make up such an add-on program

Technically, an extension is little more than a small website packed into a zip file. It’s built from the same building blocks as ordinary web pages: HTML for structure, CSS for appearance, and JavaScript for logic. On top of that comes a text file called the manifest. It states the extension’s name, which files belong to it, and which permissions it requires.

When installing, the browser reads this manifest and shows you the requested permissions. After that, typically two kinds of code run. So-called content scripts get loaded into every matching web page and can read and change its content. A background script keeps running independently of individual pages and handles tasks such as fetching data from the network.

A common misconception: an extension and an app are not the same thing. An extension can only do what the browser allows it to do, and it runs in a sandboxed area. It normally can’t reach your files on the hard drive. Its reach ends where the browser ends.

AI assistants, ad blockers, and the fight over the rules

In everyday life, you mostly encounter extensions as ad blockers, password managers, or translation aids. In recent years, AI tools have joined the mix. They overlay a chat window on the open page and summarize a long article at the push of a button. Others suggest phrasing while you write emails or explain highlighted passages of text.

In the news, extensions tend to show up in two contexts. The first is security: Google and Mozilla regularly remove hundreds of extensions from their catalogs because they were secretly collecting browsing data. The second is a power struggle. With a new rule version called Manifest V3, Google has changed how extensions are allowed to block content. Critics say this weakens ad blockers in particular — and Google makes its money from advertising.

In practical terms, this means: install few extensions, and check the list in your browser menu now and then. Anything you haven’t used in months, you can remove. Every extension less is one less program reading along as you browse.