Browser Support

We seek to provide usable experiences of our most important web content to all user agents. But newer browsers are far more capable than older browsers, and the capabilities they provide are valuable to developers and site visitors. We will take advantage of modern browser capabilities. Older browsers will have a different experience of the website than newer browsers. We will strike this balance by generally adhering to the core principles of Progressive Enhancement:

  • Basic content should be accessible to all web browsers

  • Basic functionality should be accessible to all web browsers

  • Sparse, semantic markup contains all content

  • Enhanced layout is provided by externally linked CSS

  • Enhanced behavior is provided by unobtrusive, externally linked JavaScript

  • End-user web browser preferences are respected

Some website experiences may require us to deviate from these principles – imagine a marketing campaign page built under timeline pressure to deliver novel functionality to a particular locale for a short while – but those will be exceptions and rare.

Browser Support Matrix (Updated 2022-07-06)

We deliver enhanced CSS & JS to browsers in our browser support matrix (below). We deliver degraded support to all other user agents, except legacy IE browsers, which get basic support.

The following browsers have enhanced support:

  • All evergreen browsers (Firefox, Firefox ESR, Chrome, Safari, Edge, Opera, etc.)

The following browsers have degraded support:

  • Outdated evergreen browser versions.

  • IE11 & IE10.

The following browsers have basic support:

  • IE9 and below.

Delivering basic support

On IE browsers that support conditional comments (IE9 and below), basic support consists of no page-specific CSS or JS. Instead, we deliver well formed semantic HTML, and a universal CSS stylesheet that gets applied to all pages. We do not serve these older browsers any JS, with the exception of the following scripts:

  • Google Analytics / GTM snippet.

  • HTML5shiv for parsing modern HTML semantic elements.

  • Stub Attribution script (IE8 / IE9).

Conditional comments should instead be used to handle content specific to IE. To hide non-relevant content from IE users who see the universal stylesheet, a hide-from-legacy-ie class name can also be applied directly to HTML:

<p class="hide-from-legacy-ie">See what Firefox has blocked for you</p>

Delivering degraded support

On other legacy browsers where conditional comments are not supported, developers should instead rely on feature detection to deliver a degraded experience where appropriate.


The following feature detection helpers will return true for all browsers that get enhanced support, but will also return true for IE11 currently, even though that has now moved to degraded support. The reason for this is that whilst many of our newer products don’t support IE at all (e.g. Mozilla VPN, Firefox Monitor, Firefox Relay), we do still need to provide support so that IE users can easily download Firefox. We can decide to update the feature detect in the future, at a time when we think makes sense.

Feature detection using CSS

For CSS, enhanced experiences can be delivered using feature queries, whilst allowing older browsers to degrade gracefully using simpler layouts when needed.

Additionally, there is also a universal CSS class hook available that gets delivered via a site-wide JS feature detection snippet:

.is-modern-browser {
    /* Styles will only be applied to browsers that get enhanced support. */

Feature detection using JavaScript

For JS, enhanced support can be delivered using a helper that leverages the same feature detection snippet:

(function() {
    'use strict';

    function onLoad() {
        // Code that will only be run on browsers that get enhanced support.

The site.isModernBrowser global property can also be used within conditionals like so:

if ( {
    // Code that will only be run on browsers that get enhanced support.

Exceptions (Updated 2019-06-11)

Some pages of the website provide critical functionality to older browsers. In particular, the Firefox desktop download funnel enables users on older browsers to get a modern browser. To the extent possible, we try to deliver enhanced experiences to all user agents on these pages.

The following pages get enhanced experiences for a longer list of user agents:

  • /firefox/

  • /firefox/new/

  • /firefox/download/thanks/


An enhanced experience can be defined as a step above basic support. This can be achieved by delivering extra page-specific CSS to legacy browsers, or allowing them to degrade gracefully. It does not mean everything needs to look the same in every browser.