by I’m currently looking at this. The fix to test_accept_header.html is straightforward, but it led me to run a full mochitest suite and that showed some less-obvious errors. In particular image/test/mochitest/test_bug496292.html is failing, but it took me awhile (I got sidetracked by bug 1682709) to track down why.
To hopefully avoid another backout, I’ve just replaced all the instances of “image/webp,*/*” (our previous Accept header) in our test code with “image/avif,image/webp,*/*”. I’ll run another try and hopefully that one will be green.
