I know they're annoying as shit but how trivial would it be to add webp support to the image attachments? Most OSs are now supporting them natively now and even adobe can export to webp now.
If you really want an un-fun job, you could probably save a fair bit of diskspace and bandwidth converting all images to them but I don't think it's worth the effort. Just be nice to be able to embed them easier as Google and a lot of article sites and shot offer them by default.
Edit also are you testing avatar shapes or does xenforo just hate Firefox? I'm seeing thumb versions in square and circle form in some places, and square and octogon (I assume a broken circle) in some threads. If you're experementing I like the squares but circles could work on profiles.
Edit is there a reason for the negative reactions? Just because webp is cancer, because it's been asked before and I'm a faggot, a combo of the two or something else?