@frehi I've worked on Chromium quite a bit, in terms of patching and building, so I'm speaking from that experience. I don't know much about Firefox in Debian.
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@frehi I've heard some more details: Debian's Chromium maintainer actually got it to use the system libwebp https://salsa.debian.org/chromium-team/chromium/-/blob/master/debian/control
And Mozilla maintains the Firefox packages in Debian, they decided to not use the system libwebp, though their build system supports it.
