Firefox 156 Uses Up to 20 Times Less Memory on Big JPEG Photos

Firefox 156 stops unpacking huge photos at full size just to show them small - and on a big enough image that is the difference between 20 units of memory and one.

Websites are full of photos that are far bigger than the space they are shown in.

Until now, your browser opened every one of them at full size first, then shrank the result.

Firefox 156 stops doing that, and the saving is larger than it sounds.

What actually changed

A JPEG is decoded in blocks, and the library Firefox uses for that job, libjpeg-turbo, has always been able to produce those blocks at half, a quarter or an eighth size as it goes.

Firefox never used it. It decoded the whole picture at full resolution into memory, then handed that to a separate downscaler.

Now it asks for the small version up front and only fine-tunes the last step. The picture you see is the same; the full-size copy in the middle simply never exists.

The numbers, and what they are measuring

Mozilla benchmarked the change on a single 4992 by 4272 photo, which is roughly what a modern camera produces.

Shown at an eighth of its size, that image decoded about 2.4 times faster and used around a twentieth of the memory. At a quarter it was about twice as fast, and at half, about 1.8 times.

So the headline saving is real, but it is the best case on a very large file. A photo that is already close to the size it is displayed at gains nothing, because there is nothing to skip.

When it quietly does not apply

This is the part the release notes leave out, and it explains why some pages will feel exactly the same as before.

That last one is not fussiness. The edge of a JPEG is padded out to fill the final block, and at reduced scales that padding can smear into pixels you can actually see, so Firefox leaves those images alone rather than risk a visible seam.

In practice that still covers an enormous amount of the web, because most cameras and most export tools produce dimensions that fit.

You do not have to switch it on

The work landed behind a preference during testing, and a second change flipped that preference on for everyone before 156 shipped.

There is nothing to enable in about:config and nothing to install. Update Firefox and it is already running.

Also in this release

The built-in PDF viewer now starts up to 45 percent faster, because Firefox loads its background worker earlier instead of waiting for you to open a document.

There is also an audio fix worth knowing about: FLAC playback that failed on some sites, Bilibili among them, now works again.

The rest is housekeeping - Picture-in-Picture subtitles, bookmark folders in the sidebar, sites that would not load with DNS over HTTPS switched on, and the usual security patches.

Why this matters beyond Firefox

JPEG is thirty years old and still the format everything opens, which is exactly why it is worth optimising rather than replacing.

Newer formats do compress harder - our comparison of AVIF, WebP and JPEG puts AVIF at roughly half the file size - but none of them has JPEG's reach, and a page full of JPEGs is still the normal case.

Making that normal case cheaper on an old laptop with 8GB of RAM is a bigger win than any format change nobody has adopted yet.

If you would rather not wait for a browser to do the work, a desktop viewer such as JPEGView has handled large photos this way for years.

Nothing about your photos changes - Firefox just stopped doing the expensive half of a job it was throwing away anyway.

Download Mozilla Firefox and see how your usual image-heavy pages behave, or tell us whether you noticed on an older machine.