The favicons.sqlite db is growing a bit larger than expected, we have a 1,20% of users with a db larger than 50MB, and evolution shows it's growing in general. Since Activity Stream uses 96px icon (192px in hidpi), and that looks large enough to cover most UI needs, we could drop the 256px icons support. On my db this would save about 5MB of data we are not using. I'd not remove existing icons, I'd just remove the support for that size, existing icons would be expired naturally after a week.

Ursula, are you aware of any reason or close future usage for which 192px wouldn't be enough?

I can't think of any usage with larger than 192px x 192px favicons. Since our top sites are 96px x 96px as you mentioned, 192px should be more than enough, and our highlights do not show icons anymore. In the future we'll want to show top sites with icons on an about:library page, but those should follow the same rules as Activity Stream icons, and will not be larger than 96px.

Comment on attachment 8992382 [details] Bug 1475500 - Reduce maximum favicon size from 256 to 192px. https://reviewboard.mozilla.org/r/257242/#review264132 Easy enough

Pushed by mak77@bonardo.net: https://hg.mozilla.org/integration/autoland/rev/8b01a2a2af4d Reduce maximum favicon size from 256 to 192px. r=adw