How Firefox’s New Accessibility Engine Supercharges Performance

In the early days browsers ran as a single OS process, making the Accessibility Tree easy for assistive technologies to access.

As browsers grew in complexity, they adopted multi‑process architectures and sandboxed web‑content processes, which significantly limited direct access to the Accessibility Tree.

Firefox’s accessibility engine supplies screen readers and other assistive tools with the information they need, but performance delays in the old architecture could make browsing a burden.

Because the existing engine was slow, overly complex, hard to maintain, and required different implementations per operating system, the Firefox team launched the "Cache the World" project to rebuild it.

The new goal is to make Firefox faster when used with screen readers, especially on Windows. Official benchmarks show substantial speed gains, such as opening a Gmail message in 100 ms versus 200 ms before, and reducing large‑file load times from 175 seconds to 15 seconds.

For example, opening an email now takes 100 ms (down from 200 ms), and closing it back to the inbox takes 150 ms (down from 410 ms). Loading a large file drops from 175 s to 15 s, and a 12,000‑row spreadsheet loads in 6 s instead of 128 s.

The redesigned architecture asynchronously pushes the accessibility tree from each web‑content process to the main UI process, serving queries from a cache without inter‑process calls. Updates are also pushed asynchronously, allowing the team to control buffer updates and eventually remove about 20,000 lines of OS‑specific code.

The new engine is already enabled in Firefox Nightly and is slated for the Firefox 110 test channel for Windows and Linux in January 2023, with a phased rollout in versions 111‑112, while the macOS version will follow later.