Firefox 151 landed on May 19 with a broad set of platform and developer features. The release adds Web Serial API support for communicating with microcontrollers and development boards directly from the browser, container style queries for CSS, and Document Picture-in-Picture API. On the user side, the built-in PDF viewer gains the ability to merge multiple PDFs, the New Tab page gets wallpaper customization, and private session data can now be cleared without closing the private window. Enhanced fingerprinting protection ships with a reported ~14% reduction in identifiable browser profiles. For web developers, container style queries and Document Picture-in-Picture reaching Firefox 151 stable means both APIs now have cross-browser support in current releases, making them safer to adopt in production. Web Serial API was already available in Chromium-based browsers, so its arrival in Firefox closes a notable gap for hardware-adjacent web tooling. No breaking changes are flagged in this release.