That’s intentional actually. If there is no topic image, it shouldn’t waste too much space in the sidebar. So the empty/fallback slot is smaller – at least in the sidebar.
Okay, I see the problem. The comment counter is pushing the avatar to the side in Firefox.
Until I fix that in my templates, try adding this to your custom.css:
.ST_avatar_wrapper_sidebar_withimage {
clear: both;
}