Why does my currently active tab group change when I close tabs?

I think it's a bug, but I can't figure out where to post a bug report. I use tab groups (Firefox 5.0), and love them. But often, when I close a tab in one group, I get automatically switched to another tab group. More rarely, Firefox rearranges which tabs are in which groups when this happens. This usually seems to happen when I'm on a pinned "App Tab" (Gmail), and then choose to close one of the tabs via clicking its X with the mouse. I currently have 6 tab groups, all but one of which are named.