How it works
An extension is a bundle of HTML, CSS and JavaScript described by a manifest.json file. Content scripts run inside the pages you visit and can read or change them, a background service worker reacts to browser events, and a popup, side panel or options page gives it a small interface. Each ability (tabs, storage, access to certain sites) must be declared as a permission.
Chrome, Edge, Firefox, Opera and Safari share the WebExtensions API, so one codebase can target all of them with small changes; Safari needs the extension wrapped in a Mac or iOS app with Xcode. Chrome requires Manifest V3, which replaced long-running background pages with service workers and limited how extensions can block network requests. Firefox supports Manifest V3 too but kept the older blocking API. Stores review each submission, and broad permissions such as reading every site draw closer scrutiny.
Browser extensions pros and cons
Pros
- Adds features right inside the websites people already use
- Built with ordinary web skills: HTML, CSS and JavaScript
- One codebase can reach Chrome, Edge, Firefox and Safari
- Cheap to publish, with store distribution and automatic updates
Cons
- Store reviews can delay releases or reject broad permissions
- Manifest V3 limits some background and network-blocking features
- Websites change their pages, which can break content scripts
- Safari needs an Xcode wrapper and a paid Apple developer account
When to use Browser extensions
Pick it when
- Productivity tools that work on top of other sites, such as email or a CRM
- Companions to your own web app, such as clipping or quick capture
- Internal tools a company installs on staff browsers
Skip it when
- The feature works just as well as a normal website or PWA
- It needs deep system access that browsers do not allow
Browser extensions pricing
One-time fee
Chrome Web Store charges a one-time $5 developer registration fee. Firefox Add-ons and Microsoft Edge Add-ons are free to publish; Safari needs the Apple Developer Program ($99 a year).
Browser extensions pricing page (opens in a new tab)Approximate, checked September 2026.What the other tools cost
Related terms
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