Mozilla has released Firefox 155, introducing networking changes designed to reduce connection delays, alongside new privacy indicators and Smart Window improvements.
The new release adds Happy Eyeballs v3 and QUIC v2 support for HTTP/3, both aimed at helping Firefox establish faster connections to websites.
Happy Eyeballs v3 changes how Firefox chooses between available network paths when opening a website. Instead of committing to a single combination such as IPv4, IPv6, HTTP/2, or HTTP/3 and waiting for it to fail, Firefox can attempt multiple connection options in parallel and use the one that responds first.
This approach can reduce delays caused by poorly configured IPv6 networks, unavailable HTTP/3 endpoints, or other connectivity problems. Happy Eyeballs v3 is still being developed as an IETF draft, and Mozilla indicates that Firefox 155 currently supports it only on some platforms.
Mozilla, the organization behind Firefox, develops the browser as one of the largest remaining independent alternatives to Chromium-based browsers. Firefox also continues to support extensions based on capabilities no longer available in Chrome, following Google's phase-out of Manifest V2 extensions.
Firefox 155 also adds support for QUIC version 2, standardized as RFC 9369, for HTTP/3 connections. When a compatible server offers QUIC v2 through version negotiation, Firefox can select it without performing additional negotiation round trips.
HTTP/3 runs over QUIC rather than TCP and is designed to establish secure connections with fewer network exchanges, particularly benefiting high-latency or unreliable connections. Firefox 155's compatible version negotiation can further reduce connection setup overhead when the browser and server initially use different QUIC versions.
Mozilla says some websites may load slowly or fail entirely on networks that block UDP, including corporate environments where firewalls or TLS inspection systems prevent QUIC traffic. Administrators can temporarily disable network.http.happy_eyeballs_enabled through Firefox's Preferences enterprise policy. Mozilla plans to fix the problem in Firefox 155.0.1 by September 8.
Elsewhere, Firefox now displays the number of blocked trackers in the address bar as part of a progressive rollout. Users can hide the counter without disabling tracker protection. Mozilla has also changed captive-portal detection from detectportal.firefox.com to firefox-portal-detection.com, requiring organizations with network allowlists to update their configurations.
Smart Window is now available to users in the United States, Canada, and France, adding personalized conversation-resume suggestions, faster assistant responses using Exa web search citations, and improved automatic tab grouping.
Other changes include reorderable Containers, Nintendo Switch Pro controller support on Windows, improved PDF print scaling, and restrictions preventing websites from opening mailto: links without explicit user interaction.
Firefox 155 also fixes background audio interruptions, Linux sleep prevention, MacBook fullscreen letterboxing, and text-copying problems on X and YouTube.
Firefox can be upgraded to the latest version from within the browser’s settings menu, or, when on Linux, from the distribution’s package manager.
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