Mozilla developers have been working to "modernize the core experience" so that it's cleaner, more inviting, and easier to use. There are navigation improvements, streamlined menus, an enhanced tab design, and more cohesive visuals.
Aside from the UI/UIX improvements with Firefox 89, the beta release also continues with privacy/security improvements, support for the Event Timing API, and other enhancements.
More details and downloads on the Firefox 89 Beta ahead of next month's stable release can be found at Mozilla.org .