Mozilla’s Firefox 156 is available today, and its two headline performance improvements are the result of specific engineering choices worth understanding: a PDF viewer that pre-warms its background worker to eliminate cold-start delay, and a JPEG decoder that does its downscaling inside the decompression stage itself — before a full-resolution image is ever placed in memory. The full list of changes is in the Firefox 156 notes published today by Mozilla.

This is the second Firefox release since Mozilla moved to a twice-monthly release schedule, a cadence that began with Firefox 155 on September 1.

PDF.js Worker Pre-Loading Eliminates the Cold-Start Penalty

Firefox’s built-in PDF viewer — powered by PDF.js, the browser’s open-source JavaScript rendering library — now starts up to 45% faster than it did in Firefox 155.

The improvement comes from a change in when the background worker thread gets created. In every previous version, the PDF.js rendering worker — the thread responsible for parsing and rasterizing document content — was only spawned when a user actually clicked on a PDF link. Thread creation carries an overhead cost: the operating system must allocate resources, the JavaScript engine must initialize a new execution environment, and PDF.js must load its parsing modules. None of that happened in advance because Firefox didn’t know a PDF was coming.

Firefox 156 removes this cold-start delay by pre-loading the PDF.js worker earlier in the browser’s startup sequence, before any user action. When a user clicks a PDF link now, the worker is already running. For anyone who regularly opens PDFs from the browser — government forms, research papers, invoices, receipts — the document appears nearly instantaneously instead of after a perceptible delay.

What IDCT Scaling Actually Does to JPEG Memory

The second major change addresses how Firefox handles large JPEG images on web pages, and the mechanism matters because it explains why the savings are so dramatic.

Standard JPEG decompression works by reversing the encoding process: stored frequency coefficients are fed through an Inverse Discrete Cosine Transform (IDCT), which reconstructs pixel values in 8×8 blocks. In Firefox’s previous implementation, that process always reconstructed the image at its full native resolution — even when the final display size was much smaller. A 4K photograph embedded in a blog post at 800 pixels wide still required a full 4K-resolution pixel buffer in memory before the compositor could scale it down to fit.

Firefox 156 adopts libjpeg-turbo’s IDCT scaling, which changes this at the mathematical level. The IDCT can be computed with only the low-frequency coefficients, producing a smaller output block directly — at 1/2, 1/4, or 1/8 the original dimension — without constructing the full image first. A 3,840-pixel-wide JPEG that a CSS rule will display at 480 pixels can be decoded to that target size in a single pass. The full-resolution 3,840-pixel buffer is never allocated.

The result, according to benchmarks in Mozilla’s bug tracker, is up to 20 times less memory consumed during very large image loading, and decoding speed up to twice as fast. Because image decoding was already quick on modern hardware, most users on recent machines will not notice a subjective difference while scrolling. The gains compound meaningfully over a typical browsing session full of image-heavy pages — and they carry particular weight on hardware where RAM and CPU headroom are limited.

On budget laptops, older machines, Chromebook-class ARM devices, and entry-level Windows hardware, JPEG decoding is a genuine bottleneck on image-heavy pages. Peak memory pressure from decoding multiple large images simultaneously can trigger browser-level garbage collection pauses and dropped frames. Cutting peak allocation by up to 20 times directly reduces that pressure and can mean the difference between a page that scrolls fluidly and one that hitches.

VPN Connectivity Bug Patched

Also fixed in Firefox 156: a regression introduced in Firefox 155 that caused the browser to lose all network connectivity for some users who had Mozilla’s built-in free VPN service enabled. Users affected by this bug — where enabling the integrated VPN silently broke all browser traffic — should confirm they have updated before troubleshooting network issues.

Do Firefox’s Sponsored Address-Bar Suggestions Affect You?

Starting with Firefox 156, users in France, Germany, and Italy will see sponsored suggestions appear in the browser’s address bar — a feature already live in other regions.

Users who prefer a clean address bar can disable this via Settings > Search > Firefox Suggest > Suggestions from Sponsors. The setting does not affect other search suggestions; it is specific to the sponsored entries sourced through Mozilla’s advertising relationships.

Bug Fixes Worth Noting

Firefox 156 also ships a cluster of bug fixes addressing real-world workflows, all documented in the Firefox 156 release notes:

Dragging an image in a rich-text editor no longer replaces it with a long run of text. High-sample-rate FLAC audio files embedded in MP4 containers — which failed on sites including the Chinese video platform Bilibili — now play correctly. Subtitles in Picture-in-Picture windows load properly when captions are enabled after the PiP window is already open. Bookmarks no longer escape their folder when dragged in a group via the sidebar. Some sites that failed to load when DNS over HTTPS was active have been repaired. A Windows-specific issue where Firefox could prevent an auto-hiding taskbar from appearing is also resolved.

macOS users gain one new capability: Firefox can now open automatically at system startup, configured under Settings > General > Startup.

Separately, Windows devices running ARM64 processors — including Snapdragon-based laptops and Surface Pro X machines — can now use hardware H.264 decoding for WebRTC video calls instead of falling back to software, cutting CPU load during conferences.

Security Fixes in Firefox 156

The release ships security advisory MFSA2026-90, addressing more than 40 security CVEs including high-severity use-after-free bugs across several components including Audio/Video Web Codecs, privilege escalation issues in the Graphics/CanvasWebGL component, a sandbox escape in the DOM component, and a privilege escalation in Firefox for Android. Mozilla has changed how it publishes advisories: individual memory-safety bugs now each receive their own CVE entry rather than being grouped under a single catch-all CVE.

What’s Coming in Firefox 157

The next Firefox release — version 157 — is expected on September 29, 2026, approximately two weeks from today. That release is scheduled to enable Mozilla’s “Nova” visual redesign as the default interface for all users. Nova introduces a modernized aesthetic with rounded UI elements and refined gradients, bringing Firefox’s look into alignment with broader design trends in Chrome and macOS Sonoma. Users who want to preview Nova now can enable it in current stable releases by setting browser.nova.enabled to true in the about:config page.

How to Get Firefox 156

Firefox 156 is available today from Mozilla’s direct download page and rolling out as an automatic update to existing users on Windows, macOS, and Linux. Ubuntu users running Firefox from the Snap Store will receive the update automatically in the background; a restart prompt will appear when the update is ready.

Frequently Asked Questions

What’s the technical reason Firefox 156 opens PDFs so much faster?

Firefox’s PDF viewer uses a background JavaScript worker thread to handle the heavy work of parsing and rendering documents. In previous versions, this worker was only created on demand — when you actually clicked a PDF link. Firefox 156 pre-loads it during browser startup, so the thread is already running when you need it. The cold-start penalty is eliminated, producing a startup improvement that Mozilla benchmarks at up to 45%.

Why would cutting JPEG memory use by 20 times not make pages feel noticeably faster?

On a modern machine with several gigabytes of RAM and a fast CPU, JPEG decoding is typically done so quickly that the raw speed improvement isn’t perceptible while scrolling. Where the change matters is in peak memory allocation: loading a large image no longer requires a full-resolution pixel buffer before downscaling happens. On low-powered hardware — budget laptops, older ARM devices, Chromebook-class machines — this reduction in peak memory pressure directly reduces the likelihood of garbage collection pauses and frame drops on image-heavy pages.

My Firefox built-in VPN stopped working a couple of weeks ago. Is this fix for me?

Yes, if you’re running Firefox 155 with Mozilla’s built-in VPN enabled and experiencing complete network connectivity loss, that regression was introduced in version 155 and is patched in Firefox 156. Update via Help > About Firefox or wait for the automatic update notification.

What is the Nova redesign, and when does it arrive?

Nova is Mozilla’s first major visual overhaul of Firefox since the “Proton” redesign in 2021. It introduces rounded elements, updated gradients in the browser chrome, and refined padding throughout the interface. It is currently available as an opt-in flag in Firefox 156 via about:config (set browser.nova.enabled to true), and is scheduled to become the default interface for all users when Firefox 157 ships around September 29, 2026.