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Firefox's current architecture for multi-process accessibility suffers from severe performance issues and is costly and difficult to maintain due to the massively different and specialised approaches necessary on different operating systems. In addition, it is currently impossible to support builtin Windows accessibility tools such as Narrator and Windows Speech Recognition. This project aims to re-architect our multi-process accessibility support to cache the entire accessibility trees for all content processes within the parent process.

Meeting Notes

Jamie, Morgan, and Eitan meet weekly to discuss this project. You can find the meeting notes in this google doc.


Given the large scope of the project, we are breaking the project down into quarterly milestones. Each milestone will aim to support a set of user scenarios. This roadmap is in the early stages and subject to significant change. It will be updated as milestones become clearer, with future milestones being less well defined than earlier ones.

Milestone 0: December 2021

Initial proof of concept.

All testing will be performed with the NVDA screen reader, for two reasons:

  1. Cache the World is all or nothing on Windows. That makes it easy to determine where we're at regarding real world usage.
  2. The performance benefits are most necessary and noticeable on Windows.

In milestone 0, the following capabilities will be provided:

  1. Reading and navigating a page with text, links and headings.
  2. Plain text editing: reading the line of text when focused; backspacing; moving the caret by line, word and character.
  3. Access to formatting information: font, bold/italic, etc.
  4. Access to screen coordinates on simple pages.
  5. Loading very large pages will be at least 10x faster with the cache than without.

Test Scenarios

  1. Do a Google search, navigate the results using heading navigation and follow a result link.
  2. Fill out the form for a Google Advanced Search.
  3. Go to Check the formatting of the “This is REAPER.” text (which isn’t a heading even though it should be) and confirm that the font size is reported as bigger than the paragraph of text below it.
  4. Build Gecko in the background. Do a Google search. Verify that the browser does not become unresponsive.
  5. Load Page should take < 10 sec to be usable.
  6. Open Gmail, find a message in the inbox, open it, read it, return to the inbox.
  7. Compose a message in Gmail containing text, a link, a bulleted list and a block quote. Read back through the message.
  8. Open Slack, use the quick switcher to switch to a channel, read some messages, write a message.


Note that the roadmap wasn't created until late in milestone 0, so many bugs are missing below.

Full Query
ID Summary Assigned to Status
1735955 Cached bounds all 0s for many (most?) elements on Google search James Teh [:Jamie] RESOLVED
1739050 If the focused Accessible is moved, the RemoteAccessible is recreated but focus isn't fired on it (AKA broken Google Search box on Windows + CTW) James Teh [:Jamie] RESOLVED
1741792 Cache the caret James Teh [:Jamie] RESOLVED
1742902 Fix window emulation when the cache is enabled James Teh [:Jamie] RESOLVED
1742915 Cache tag object attribute James Teh [:Jamie] RESOLVED
1742917 Implement StartOffset for RemoteAccessible and LinkIndexAtOffset for HyperTextAccessibleBase James Teh [:Jamie] RESOLVED
1746827 Crash in [@ PLDHashTable::Search | mozilla::a11y::RemoteAccessibleBase<T>::MinValue] James Teh [:Jamie] RESOLVED

7 Total; 0 Open (0%); 7 Resolved (100%); 0 Verified (0%);

Milestone 1: March 2022


The primary focus of this milestone is getting the cache working for Android. Mozilla aims to implement Fission for Android in 2022h1. Modifying the existing multi-process architecture to support Fission on Android will require significant engineering effort. Rather than investing in a solution which we will be throwing away once the cache is implemented, we will instead switch Android to use the cache and extend the cache to include functionality required by Android.

In milestone 1, the following capabilities will be provided:

  1. The cache will support GroupPosition.
  2. TextLeafRange will support word end and line end boundaries, which are needed for Android text navigation.
  3. Pivot will support navigating text using TextLeafRange, which is needed for Android text navigation.
  4. Cached screen bounds will be updated appropriately when scrolling.
  5. Android will use the cache for all functionality except hit testing.
  6. As an interim solution, Android will use the existing async IPDL mechanism for hit testing, updated to target the call at the correct document to handle OOP iframes. (Synchronous hit testing in the core cache will take longer to implement and will be done in a future milestone.)

Test Scenarios

  1. TBD: simple website reading. News site?
  2. TBD: filling a form.
  3. TBD: character/word/line navigation.
  4. With Talkback, load Navigate to the embedded video in four different ways: item navigation, explore by touch, controls navigation and Talkback search. Activate the Play button to play the video.


Full Query
ID Summary Assigned to Status
1730085 TextLeafPoint: Implement BOUNDARY_WORD_END and BOUNDARY_LINE_END NEW
1730086 TextLeafPoint: Implement HyperTextAccessible::TextBefore/AfterOffset using TextLeafPoint James Teh [:Jamie] NEW
1735970 [meta] Cache table information NEW
1737944 Implement GroupPosition for cached RemoteAccessibles Eitan Isaacson [:eeejay] RESOLVED
1739560 Update cache when line boundaries change NEW
1741793 Cache text selection NEW
1746833 Implement textual value for cached RemoteAccessibles NEW
1747172 Make Pivot text functions use HyperTextAccessibleBase NEW
1747835 Cache text input type Eitan Isaacson [:eeejay] RESOLVED
1748749 Cache ActionCount/ActionNameAt Eitan Isaacson [:eeejay] ASSIGNED
1749828 Support cached grouppos in windows and xpcom Eitan Isaacson [:eeejay] NEW

11 Total; 9 Open (81.82%); 2 Resolved (18.18%); 0 Verified (0%);

Milestone 2: June 2022

TBD. Opt-in user preview. Enable in Nightly?

Milestone 3: September 2022

TBD. Beta experiment? Release experiment?


Full Query
ID Summary Assigned to Status
1234121 Cache active item from child process in parent and remove sync call for FocusedChild NEW
1694573 Make AccEvent use new Accessible base class Morgan Reschenberg [:morgan] NEW
1730090 TextLeafPoint: Implement BOUNDARY_PARAGRAPH NEW
1730095 Implement Mac text navigation using TextLeafPoint/Range NEW
1730868 TextLeafPoint: Better handling for inline-block for line boundaries NEW
1733188 [meta] Notify of state changes for caching purposes NEW
1733417 [meta] Cache relations NEW
1733486 [meta] Cache text NEW
1735101 Cache text screen bounds NEW
1737919 TextLeafPoint: Implement support for spelling errors NEW
1739559 Update cache when text attributes change NEW
1743749 Clean up use of atoms for cache keys NEW
1743966 Implement ScrollTo for cached RemoteAccessible on Windows NEW
1743967 Implement DoAction for cached RemoteAccessible on Windows Eitan Isaacson [:eeejay] NEW
1743968 Cache the display object attribute NEW
1748393 Update cache when title of document changes NEW
1748755 Support live regions with the cache enabled on Windows James Teh [:Jamie] NEW

17 Total; 17 Open (100%); 0 Resolved (0%); 0 Verified (0%);