This is expected, see 1882462. We delete layer manager instead of keeping it and pile up hundreds of MB per popup. It can be changed when Bug 1654938 is fixed (provide shared web render content for windows).
Bug 1883308 Comment 2 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
This is expected, see Bug 1882462. We delete layer manager instead of keeping it and pile up hundreds of MB per popup. It can be changed when Bug 1654938 is fixed (provide shared web render content for windows).