Firefox’s most popular and innovative browser extensions of 2021

About one-third of Firefox users have installed an add-on—be it an extension to equip Firefox with a custom feature or a visual theme to personalize their browsing experience. 

What kind of extensions did we use most in 2021? And what are some of the new, innovative extensions to emerge this year? Let’s find out!

Two ad blockers vie for most popular in 2021

Note we’re using ‘average daily users’ (ADU) across the span of 2021 as the metric to determine “most popular.” We feel ADU offers the most accurate reflection of popularity because it captures a year-long aggregate of active usage.

2021 has two clear contenders for top extension, both of which happen to be ad blockers: Adblock Plus averaged 6,134,231 daily users while uBlock Origin averaged 5,011,974 over the course of the year. 

Adblock Plus by Adblock Plus

One of the most popular free ad blockers for Firefox. Block annoying ads on sites like Facebook, YouTube and all other websites.

Adblock Plus blocks all annoying ads, and supports websites by not blocking unobtrusive ads by default (configurable).

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This extension is not currently available.

As 2021 progressed, however, uBlock Origin gradually closed the gap and if current usage trends continue, may take the top spot sometime in early 2022. 

uBlock Origin by Raymond Hill

Finally, an efficient wide-spectrum content blocker. Easy on CPU and memory.

You'll need Firefox to use this extension
Download Firefox and get the extension

This extension is not currently available.

(Want to learn more about other great ad blockers? See our guide What’s the best ad blocker for you?)

Over the past couple of years, people have become increasingly concerned about their online privacy and more Firefox users are taking steps to protect their data. The popularity of many of 2021’s top extensions like Mozilla’s own Facebook Container (1,740,395 ADU) and Ghostery (1,167,938 ADU)—both designed to protect against invasive tracking—is testament to that.

In Russia, 60% of Firefox users have installed an add-on—by far the highest regional percentage.

The most popular theme Firefox users installed in 2021 is the simplistically beautiful Matte Black (Red)

Matte Black (Red) by Elijah Lopez

A modern dark / Matte Black theme with a red accent color.
Click my name for more accents (request if not available).

You'll need Firefox to use this theme
Download Firefox and get the theme

This extension is not currently available.

Most innovative extensions of 2021

The Firefox Recommended Extensions program is a curated list of third party extensions that meet Mozilla’s highest standards of security, utility, and user experience. The curatorial process involves a collaborative project between Firefox editorial staff and leaders in the add-ons community who participate on the Recommended Extensions Advisory Board. Here are some particularly innovative extensions that joined the Recommended program in 2021. 

Tab Stash

Do you often find yourself drowning in a sea of open browser tabs? If so, this extension is like someone tossing you a life vest. Just click the Tab Stash toolbar button and presto—all your open tabs get saved as temporary bookmarks, which offers unique possibilities. 

Tab Stash by Josh Berry

A no-fuss way to save and organize batches of tabs as bookmarks. Clear your tabs, clear your mind.

You'll need Firefox to use this extension
Download Firefox and get the extension

This extension is not currently available.

With tabs temporarily saved as bookmarks (listed in a foldaway sidebar menu), you’re free to treat them as either easily navigable links to your previous open tabs, or save them permanently as individual or grouped bookmarks. Firefox Sync users will automatically have their Tab Stash bookmarks synced to other devices. 

Tab Stash elegantly organizes tab overload.

“I have tried probably 10 – 15 tab management extensions over the years, for both Firefox and Chrome. This is the best one I’ve ever used. It’s simple and does the job.

Firefox user jonassa


Are there websites in your life that have content you love but a look you can’t stand? If so, Stylebot can help. This extension gives you the power to dramatically alter the way any website looks and you don’t need to be a pro level designer or tech whiz to do it. 

Stylebot by Ankit Ahuja

Change the appearance of the web instantly

You'll need Firefox to use this extension
Download Firefox and get the extension

This extension is not currently available.

Once you’ve got Stylebot installed on Firefox, you’re free to access it from the browser toolbar anytime you’re on a website you want to change. Just open Stylebot’s interface and click around its options that let you change everything from font sizes to color schemes to content layout. Other key features include…

  • Auto-save changes. No need to worry about losing the new look. Any changes you make are automatically saved and recalled on subsequent visits. 
  • Magic editor lets you make “Readability” changes to web pages (i.e. strip away distracting page clutter) with a couple simple clicks. 
  • Code editor gives advanced users with CSS coding skills extreme control over web aesthetics.


A derivative of the legendary Tree Style Tab extension, Sidebery builds on the concept of organizing your open tabs in a cascading “tree” format tucked away to the side of your browser. This is a fantastic extension if your online work involves constant maneuvering between gobs of open tabs. 

Sidebery by mbnuqw

Tabs tree and bookmarks in sidebar with advanced containers configuration.

You'll need Firefox to use this extension
Download Firefox and get the extension

This extension is not currently available.

Sidebery reimagines tree tab management beautifully with an elegant and economical layout, but what gives this extension innovative distinction is its collection of assorted complementary features, like… 

  • Containers management. If you use containers like Facebook Container or Multi-Account Containers to separate parts of your online life, Sidebery makes it easy to assign tabs and bookmarks to specific containers. 
  • Simple bookmarking. Drag-n-drop links and tabs to create bookmarks on the fly.
  • Right-click mouse controls. Select multiple tabs or bookmarks by right-clicking and scrolling over all the stuff you want to grab and sort. 
  • Snapshots gives you a clean overview of your open tabs. 
  • Color customization. Change the look of your labels, backgrounds, titles, and more.

“Best all-in-one Tabs manager + Groups manager + Containers manager + Bookmarks. Thank you so much.”

Firefox user A.Jamal


You don’t have to be complicated to be innovative. automaticDark simply changes the visual theme and/or color complexion of Firefox based on time of day. It’s not only fun, it can be useful to shift your browser’s color orientation to make the web more readable in different natural light settings. 

automaticDark - Time-Based Theme Changer by simonkhzhang

Automatically change Firefox's theme based on the time.

You'll need Firefox to use this extension
Download Firefox and get the extension

This extension is not currently available.

Divided into Daytime and Nighttime modes (which will automatically transition at dawn and dusk, or you’re free to customize the times you want transitions), you can choose day and night themes from the extension’s suite of preset themes (e.g. “abstract,” “lush”) or select custom themes you’ve already installed from the many thousands of themes available

Straightforward settings make it easy to customize automaticDark.

“Thank you so much for making this, this is something I’ve wanted for ages.”

Firefox user Jambon

And thank you for reading our 2021 spotlight on the most popular and innovative Firefox browser extensions of the year. Feel free to explore more ways to customize Firefox at