Signal / 005 — Firefox Nova redraws the frame, and themes need a second look

Firefox 157 landed on September 29 with Nova, Mozilla’s largest interface refresh since Proton in 2021. The headline is visual—a cleaner browser frame, a more central sidebar, and vertical tabs treated as a first-class layout—but the edge cases belong to theme authors.

The frame changed underneath the theme

Mozilla says existing themes still load, and most colors and images continue to work. Placement is the catch. Nova lets the theme background continue beneath the sidebar and vertical tabs, while the selected tab no longer gets Proton’s automatic shadow. A theme that depended on that shadow may now lose contrast.

The practical fix is explicit configuration: set tab_selected and tab_line, define sidebar and sidebar_text, and test hover colors against both toolbar and tab-strip backgrounds. Image-heavy themes should inspect the backgrounds_area property. Choosing top_toolbars can keep a wide banner from stretching awkwardly down a vertical panel.

A small compatibility audit beats a surprise

Mozilla’s checklist covers horizontal and vertical tabs, open and hover-expanded sidebars, private windows, compact density, and both light and dark system modes. The company also notes that the hover-expanded sidebar may miss a theme background in Firefox 157; Mozilla says that fix is planned for Firefox 158 on October 13.

MDN confirms Firefox 157’s September 29 release. Theme developers can read Mozilla’s Nova compatibility guide and compare its worked examples before pushing an update.

Nova is a reminder that customization APIs can stay technically compatible while the canvas around them moves. Escape the default—but test the new default first.

AI-assisted summary, reviewed against Mozilla’s primary documentation.