Moving this to general since the bugs will span tabbrowser and toolkit code as well as changes to other frontend modules.

Description:
Please provide an explanation of the feature or change. Include a description of the user scenario in which it would be used and how the user would complete the task(s).
Screenshots and visual UI specs are welcome, but please include sufficient accompanying explanation so that blind members of the accessibility team are able to understand the feature/change.

Figma spec is here. The product requirements are here.

How do we test this?
If there is an implementation to test, please provide instructions for testing it; e.g. setting preferences, other preparation, how to trigger the UI, etc.

Using the tab context menu with either one or two tabs selected, select the "Add split view" option. For one tab selected, it will open an about:opentabs page into a split content area (with that tab and the initially selected tab).
With two tabs selected, it will create a split content area with those selected tabs. The content areas can be resized and multiple splitviews of tabs can exist. Please see the product requirements for more details on functionality (and need info me for any questions or find us on the #splitview slack channel).

When will this ship? 149
Tracking bug/issue: This bug
Design documents (e.g. Product Requirements Document, UI spec):
Engineering lead: sclements
Product manager: asafko

The accessibility team has developed the Mozilla Accessibility Release Guidelines which outline what is needed to make user interfaces accessible:
https://wiki.mozilla.org/Accessibility/Guidelines
Please describe the accessibility guidelines you considered and what steps you've taken to address them:

We've sought input from the design stage and during implementation of some of the specifications for meeting accessibility guidelines.

Describe any areas of concern to which you want the accessibility team to give special attention:

Creating and removing splitviews, is that clear and easy to do? Is navigating between tabs in a splitview and the multiple content areas as implemented make sense? Is there more we can do to improve the experience for screen reader users or keyboard users, etc.