Here is how I reproduce, no filter subscription needed:
1. open about:memory
2. open http://www.google.com
3. right-click on "google" the image, the context menu pop-up
4. close google tab, observe about:memory
5. the http://www.google.com compartment will stay there forever... until
6. you trigger another pop-up:
- a) it's an image context menu, then this tab's compartment will take the zombie role..
b) right click in blank area, the google zombie will be GC in a few second
So it seem the context menu of image hold a global object prevent GC. I am not sure whether it only the image context menu, but you'll easy fix other, if any, to locate the reason of this issue.