For some reason it is possible to move the avatar right over to the left, past the edge of the avatar.
So that then puts a black edge down the right hand side.
If you look on the right hand side of the mini image you can see the dark grey border is slightly wider.
What's odd though is this has only just started, I haven't touched my avatar for a week.
In debugging a similar issue, I think this may be a rounding error. For example, with my test image, Chrome gave me a maximum crop value of 42px, but FF let me do 43px.