Firefox/XUL and XBL Replacement
Originally, XUL filled feature gaps in the Web platform needed to build the Firefox UI. Now, the web has evolved enough that these gaps are largely closed. By more closely aligning the Firefox UI with web standard technologies, we expect to be able to move faster and better focus on performance and tooling for the web.
The work is being done in a number of separate projects. See the XUL/XBL Replacement Roadmap document for more details.
The primary way that status updates are communicated are through newsletters to firefox-dev: