While previous releases have been focused on adding privacy- and developer-centric features, v81 is a rare release that's primarily focused on improving the Firefox's user interface (UI) elements and user experience (UX) inside the browser.
By far, the crown jewel of today's release is Firefox's new browser theme, called Alpenglow.
Built around a colorful mixture of purple, pink, and black gradients, Alpenglow has become Firefox's fourth built-in theme, alongside the monochromatic Default, Light, and Dark themes.
To activate Alpenglow, users can go to Tools -> Add-ons -> Themes.
In addition, Firefox 81 also comes with another major UI change, namely in a new skin for the browser's built-in PDF viewer.
Firefox received a new UI framework in 2017 called "Photon," but the PDF component never benefited from the new UI, still remaining on the older look. Two years later is better than never, and the Firefox PDF viewer now looks like all the other Firefox pages (Settings, Add-ons, about:config), rather than looking like an abandoned add-on.
Mozilla has also added support for AcroForm [Acrobat Form Technology/PDF] to the Firefox built-in PDF reader. If PDF files support interactive form fields, Firefox 81 will let users fill in, print, and save the PDF forms right in their browser, without needing a separate PDF app.