Firefox on macOS: now smaller and quicker to install!

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2025-01-24 04:00:05

Firefox is typically installed on macOS by downloading a DMG (Disk iMaGe) file, and dragging the Firefox.app into /Applications. These DMG files are compressed to reduce download time. As of Firefox 136, we’re making an under the hood change to them, and switching from bzip2 to lzma compression, which shrinks their size by ~9% and cuts decompression time by ~50%.

If you’re familiar with macOS packaging, you’ll know that LZMA support was introduced in macOS 10.15, all the way back in 2015. However, Firefox continued to support older versions of macOS until Firefox 116.0 was released in August 2023, which meant that we couldn’t use it prior to then.

But that still begs the question: why wait ~18 months later to realize these improvements? Answering that question requires a bit of explanation of how we package Firefox…

Most DMGs are created with hdiutil, a standard tool that ships with macOS. hdiutil is a fine tool, but unfortunately, it only runs natively on macOS. This a problem for us, because we package Firefox thousands of times per day, and it is impractical to maintain a fleet of macOS machines large enough to support this. Instead, we use libdmg-hfsplus, a 3rd party tool that runs on Linux, to create our DMGs. This allows us to scale these operations as much as needed for a fraction of the cost.