Firefox 155 Ships Faster Connections and AI Browsing Option as Mozilla Tests Two-Week Release Cycle

Mozilla pushed Firefox 155 to stable channels on September 1, 2026. The release arrives with measurable gains in page-load speed. It also expands an optional AI feature called Smart Window to more users. And it marks the start of a faster update schedule that mirrors moves by Google and Microsoft.

Happy Eyeballs v3 stands out as the biggest networking upgrade. The protocol races multiple connection methods at once. It checks modern DNS records to learn what a server supports. Then it tries IPv6 and IPv4 in parallel while racing HTTP/2 against HTTP/3. The quickest successful path wins. Pages can load faster. That simple outcome matters for users on flaky networks or distant servers.

Firefox 155 also adds support for QUIC version 2. The browser now negotiates QUIC v2 for HTTP/3 connections when the server selects it through compatible version negotiation. These changes build directly on years of transport-layer work. They don’t require new hardware. They simply make existing connections more efficient.

Smart Window brings an opt-in AI-powered browsing mode. It gains access to open tabs and browsing history only with explicit user approval. The feature offers personalized resume conversation starters. It suggests better tab groupings with clearer names. An assistant inside the window responds faster and cites sources from Exa web search. For now the option reaches users in the USA, Canada and France. Mozilla positions it as one choice among private and classic windows rather than a default experience.

Other user-facing tweaks feel smaller yet practical. Containers can now be reordered in Settings. The new sequence appears everywhere containers are listed. The address bar shows how many trackers Firefox blocked on the current page. Users can hide the count if they prefer a cleaner look. Nintendo Switch Pro controllers and compatible third-party models now map correctly in web games on Windows. Directional pads and analog sticks work as expected.

Linux users receive two fixes that remove long-standing annoyances. One resolves an issue that kept systems from sleeping after extended browsing sessions. The other corrects disappearing text in Microsoft 365 Word Online and similar web apps when certain keyboard layouts were active. These changes matter to enterprise and developer audiences who run Firefox on Linux desktops.

Developer tools also advanced.

The Rules view in DevTools gathers media-feature emulation buttons into a dedicated panel opened by an @ button. That panel now includes emulation for prefers-reduced-motion. A new keyboard shortcut lets developers disable all breakpoints at once in the Debugger. The JSON Viewer handles JSON Lines documents served with appropriate MIME types or .jsonl extensions. Each line becomes a collapsible entry with line numbers. Parse errors appear inline.

CSS gains several new capabilities. The attr() function works in any property, not just content. Developers can now write rules such as width: attr(data-size px) and supply fallback values, unit identifiers or namespaced attributes. The function also operates inside container style queries. The progress() function calculates a number between start and end values for use in calc() expressions. The alpha() function adjusts color opacity while preserving the original alpha keyword option. And font-width becomes the standard property name for what was previously font-stretch, with full @font-face and computed-style support.

JavaScript adds Promise.allKeyed() and Promise.allSettledKeyed(). These static methods accept an object of promises rather than an iterable. The fulfillment object preserves the original keys so results can be read by name instead of array position. Module loading behavior changed too. Network, error or MIME-type failures no longer cache permanently. A later successful import will succeed. This adjustment helps recovery scenarios for static and dynamic imports alike.

WebAssembly receives the Compact Import Section proposal, which shrinks module size when many imports are present. The Wide Arithmetic proposal adds 128-bit operations such as i64.add128, i64.sub128 and i64.mul_wide_s/u. These instructions target cryptography and arbitrary-precision math. WebTransport sees expanded support for send groups that share bandwidth and allow relative prioritization, plus methods for keying material export, datagram writable streams and protocol negotiation.

WebRTC interoperability improves with better error reporting on data-channel failures, accurate SCTP transport details, two-byte RTP header extension IDs and corrected statistics before and after negotiation. SVG elements gain more consistent behavior. The element now exposes the same URL properties as HTML links. getBBox() honors fill, stroke, markers and clipped options. Non-rendered elements inside masks or defs return empty rectangles for bounding-client measurements.

Enterprise administrators gain the DisableLaunchOnLogin policy to control whether Firefox starts automatically on Windows login. PDF printing now enlarges documents smaller than the paper size when scale exceeds 100 percent. Captive-portal detection switched to firefox-portal-detection.com. The browser opens mailto: links only after explicit user gestures. CSS nesting receives a hard limit of 75 levels to prevent crashes from extreme recursion.

These additions arrive as Mozilla experiments with a two-week release cadence for desktop and Android. Director of Engineering Sylvestre Ledru announced the shift in July. The change takes effect with Firefox 155, which shipped on September 1 instead of the previously planned September 15. Heise Online reported that the trial does not demand twice the development speed. Features still ship when ready. Mozilla will monitor results and adjust if necessary. The move aligns Firefox with Chrome’s schedule and responds to user demand for quicker security and stability updates.

Phoronix highlighted the networking gains and Linux sleep fix in its coverage of the release. Its article noted that Happy Eyeballs v3 and QUIC v2 together target faster page loads without user configuration. The same piece listed the WebAssembly proposals and CSS progress() function among developer changes. MDN’s developer release notes provide the authoritative list of web-platform additions and DevTools updates. Those notes detail every bug fixed and API extended. Firefox’s own beta release notes captured the Smart Window rollout and controller improvements. They also documented the tracker-count indicator and container reordering.

9to5Linux covered the beta phase in mid-August and called attention to the Linux-specific fixes and PDF scaling behavior. Its preview article listed the 75-level CSS nesting cap and the new captive-portal domain. The Register examined the schedule change itself. It quoted Ledru’s mailing-list post explaining that work would not be rushed and that the experiment could be reversed.

Taken together, Firefox 155 delivers concrete performance and privacy signals in the address bar while giving developers more expressive CSS and stronger WebAssembly and transport primitives. The faster release rhythm tests whether Mozilla can sustain quality at shorter intervals. Early feedback on X focused on the speed claims and Smart Window availability in additional countries. Users who value rapid security patches may welcome the shorter cycle. Those who prefer stability might watch the first few dot releases closely.

The binaries are available now. Enterprises running ESR branches receive corresponding updates. For most desktop users the upgrade happens automatically. But the combination of faster connections, visible privacy metrics and expanded developer tools gives administrators and power users reason to test the new version promptly.