Staging groups get reset in Firefox

[LeXofLeviafan]

I'm not normally using staging groups, so I can't tell how recent this is, but now that I'm checking this functionality, it turns out that in Firefox, any of the following events causes the staging groups to be reset:

• refreshing a tab
• navigating to a different page
• switching a tab (with mouse or Ctrl+Tab)
• opening a new tab (with Ctrl+T, or Ctrl+N/Ctrl+Shift+P in a new window)

Oddly enough, switching a window (i.e. Alt+Tab) does not cause a reset (even if it's a window of the same browser).

Edit: after further inspection it turns out that the issue doesn't occur until the extension has been running for a while.