Mozilla releases Firefox 100: here is what is new

The first three-digit version of Mozilla's Firefox web browser has been released on May 3, 2022. Firefox 100 introduces a whole range of new features and improvements, including captions and subtitle support on YouTube for Picture-in-Picture, multiple language spell checking, HDR video on Mac, and hardware accelerated AV1 video decoding on Windows.

firefox 100

Firefox 91.9.0 ESR and Firefox 100 for Android are also released. All development channels have seen a version bump as well. Firefox Beta and Developer editions 101, and Firefox Nightly 102 are now also available.

Executive Summary

Firefox 100 is the first version on Windows that is signed with SHA-256. On Windows 7 devices, KB4474419 is required to install Firefox successfully.
Some sites or services may break, if they use incorrect parsing of the user-agent. Chromium reached 100 as well recently, and this issue applies to all browsers that reach this version. Mozilla has support pages for desktop and Android for users who run into compatibility issues.
Firefox 102 is the next ESR release.

Firefox 100.0 download and update

Firefox 100 is distributed via the browser's built-in updating functionality and the Mozilla website. If you are reading the article on May 3, it may not be available yet, depending on when you are reading the article.

Firefox users may run a manual check for updates to speed up the installation of the new version:

Select Menu > Help > About Firefox.

Firefox displays the current version that is installed and runs a check for updates. Any update found will be downloaded and installed automatically.

Here are the links to the official download repositories.

Firefox 100.0 new features and improvements

Picture-in-Picture video captions support

Firefox's Picture-in-Picture (PiP) feature has seen several improvements in the new release. It supports video captions and subtitles on YouTube, Prime Video and Netflix now. All it takes is to turn on subtitles in the web interface of the service to see captions and subtitles appear in the browser's Picture-in-Picture mode as well.

Additionallly, PiP supports WebVTT (Web Video Text Track) captions, which is supported by sites such as Coursera and Canadian Broadcasting Corporation.

Note: the feature was not enabled by default on a test Firefox installation. I had to set media.videocontrols.picture-in-picture.display-text-tracks.enabled to TRUE on about:config to enable the feature.

Language and spell checking improvements

Firefox will prompt users on first run of the browser if the installed language of the browser does not match the language of the operating system. Users may then pick either of the languages as the display language in the browser.

Firefox's spell checking component is now multi-lingual. Users need to install additional dictionaries in the browser to use the feature. All it takes then is to right-click on the text field and select or deselect the additional languages under Languages.

Preferred color scheme in Settings

Firefox users may now select the preferred color scheme for websites. The option is found under Website appearance on about:preferences.

Options include Firefox theme, system theme, light or dark.

Other changes

Firefox on Mac devices supports HDR video. The first site that has been added by Mozilla is YouTube, but more will follow. It requires Mac OS 11 or newer and a HDR-compatible screen. Mac users need to make sure that "Optimize video streaming while on battery" is not enabled on mobile devices, as the feature won't work in that case.
Windows users with at least Intel 11th generation processors, AMD RDNA 2 Excluding Navi 24 or GeForce 30, may benefit from hardware accelerated AV1 video decoding in Firefox 100. Mozilla notes that installation of Microsoft's AV1 Video Extension from the Microsoft Store may also be required.
On Windows systems with Intel processors, video overlay is enabled to reduce power usage during video playback.
On Twitch, performance when using the volume sliders should be improved noticeable.
Scrollbars on Windows and Linux do not take up space by default. Firefox follows the system default on Windows, which users may alter under System Settings > Accessibility > Visual Effects > Always show scrollbars on Windows 10 and 11. On Linux, users can change this in the Settings.
Firefox supports credit card autofill and capture in the United Kingdom.
Firefox ignores several referrer policies for cross-site subresource/iframe requests to improve user privacy by preventing privacy leaks. These include unsafe-url, no-referrer-when-downgrade, and origin-when-cross-origin.
On Mac OS 11 and newer, Firefox rasterizes fonts once per window; this should improve the new tab opening and tab switching performance.
Soft-reloading websites won't revalidate all resources anymore.
Non-vsync tasks have more time to run, which improves behavior on Google Docs and Twitch.
The focus indicator for links changed from a dotted outline to a solid blue outline to unify focus indicators across the browser.
Firefox can be set as the default PDF handler when setting Firefox as the default browser.

Developer Changes

WebAssembly supports exceptions that can be thrown and caught in WebAssembly or in JavaScript.
WritableStream, WritableStreamDefaultWriter, WritableStreamDefaultController, and ReadableStream.pipeTo() are now supported.
CSS media features for dynamic-range and video-dynamic-range are now supported.
Multiple Java threads can be profiled now.
Performance of deeply-nested display:grid elements is "greatly improved".
Geckoview APIs added to control the start and stop time of capturing a profile.