I don't think "Firefox just became much slower" is a very good reason; that's their fault and if they release knowing it's so much slower, they're screwing up. (If they can't do this without making the browser slower, they need to turn it off by default for those images.) There are other reasons why progressive images sort of suck, though. They're intended for slower connections--an old feature for modems--but they're just ugly and annoying for modern connections, which is probably what you have if you're viewing the full-res images here live.

That said, I wouldn't recompress interlaced JPEGs just to deinterlace them. If I'm tweaking an image for other reasons, I try to remember to switch interlacing off, though.

The image samples don't currently have support for color profiles.