Firefox 151 tightens privacy with local prompts, session reset

Mozilla released Firefox 151 with several privacy and security updates. The release adds a visible control for private sessions, expands local‑network access prompts for more users, updates anti‑fingerprinting protections in the default setting, and includes security fixes.

A new fire‑icon button appears next to the address bar when a user is in Private Browsing Mode. Activating the button clears the current private session’s data — history, cookies, cached files and other site data — and immediately starts a new private session without requiring all private windows to be closed. The control targets users who switch between normal and private windows and want to clear a session more quickly.

Firefox 151 tightens anti‑fingerprinting measures in the Standard Enhanced Tracking Protection mode. Release notes state the browser now limits device and browser information exposed to websites, which reduces the number of uniquely identifiable users by about 14% overall and by roughly 49% on macOS. The update reduces the data available to scripts that try to distinguish one browser instance from another.

Local‑network access restrictions are expanded beyond users who set Enhanced Tracking Protection to Strict. When a website attempts to communicate with devices on a user’s local network or with services running on the local machine, Firefox will prompt for permission before allowing the connection. Mozilla is rolling this behavior out progressively, so not all users will see it immediately.

The update includes several security patches. The most notable is CVE‑2026‑8953, a use‑after‑free vulnerability in the Disability Access APIs component that could lead to a sandbox escape. A use‑after‑free occurs when a program tries to access memory after it has been freed; attackers can sometimes exploit such errors to crash a program or execute code. At release, there were no reports of this bug being exploited in the wild.

Mozilla notes that several features are part of a progressive rollout. To get the protections and patches in Firefox 151, users can open Firefox, choose the menu, go to Help > About Firefox to trigger an automatic update download, and restart the browser when prompted; once updated, Firefox displays a green checkmark and the message that it is up to date.