Firefox 156 now reports `::-webkit-scrollbar` as unsupported in `@supports`, flipping negated feature queries onto standard scrollbar properties, and moves `text-box-trim` onto `::first-line` metrics.
Firefox 156 arrived on September 15, 2026, and ::-webkit-scrollbar is now reported as unsupported in @supports on every site. @supports selector(::-webkit-scrollbar) evaluates to false, and @supports not (selector(::-webkit-scrollbar)) evaluates to true (Bug 2062782).
Firefox still acts on those rules for domains listed in the layout.css.fake-webkit-scrollbar.enabled-domains pref introduced in Firefox 155. It just no longer reports support. Sites that gated standard scrollbar styling behind @supports not (selector(::-webkit-scrollbar)) now get those standard styles applied, and MDN names scrollbar-width and scrollbar-color as the replacements.
text-box-trim and text-box-edge changed too, and the difference is visible: trimming now follows ::first-line metrics, so a trimmed heading can sit differently than it did in Firefox 155, and fragmented inline boxes stop picking the wrong line. Trimmed elements also keep their border and padding. Trimming still has no effect when combined with line-clamp.
Audit your @supports not (selector(::-webkit-scrollbar)) guards first, because those blocks now apply where they previously did not, and that is the one change here that can visibly break a layout. Keep text-box-trim on the radar and re-check your trimmed headings the next time you touch them. DevTools resolves var(), attr(), and env() substitutions now; that one is free, use it when you need it.
Feature queries can now test for an at-rule rather than a property, and overscroll-behavior gains a value that hands scrolling to the next container without the bounce.
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Firefox 154 is the last gap: text-box and the sibling functions now work in every major engine, so you can delete the JavaScript and utility-class hacks doing their job.
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The v7 alpha rewrites the CSS parser, brings CSS custom properties and media queries to React Native, and adds a built-in animation adapter. No Reanimated required.
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