Exciting updates in Nightly 131 include vertical tabs and a new sidebar experience, highly anticipated by the community. To try it out, update to the latest Nightly, activate Sidebar and Vertical tabs experiments in Firefox Labs, and customize your toolbar. The new features aim to enhance context-switching and multitasking, allowing you to easily cross-reference tools and switch between tasks. Feedback on Mozilla Connect is encouraged to refine these features. Web extension developers are urged to test their extensions with the new sidebar and vertical tabs. No changes to Web Extension APIs are expected, but it’s a good opportunity to address any unforeseen issues.
https://blog.nightly.mozilla.org/2024/08/07/firefox-sidebar-and-vertical-tabs-try-them-out-in-nightly-firefox-labs-131/