I wonder to ask, have you tested your addon on Firefox with opened pinned tabs?
In my case, after checking for changes using DistillWeb there are some tabs related to Distill-addon remaining in Firefox along with and right after my previously pinned tabs.
Pinned tabs are used to place a tab at very beginning of a queue of all tabs and avoid closing it.
It is some sort of important tabs.
Maybe it’s one of the reasons why it is not closed after cheching for changes.
Generally, opening tabs (especially pinned ones) during detecting changes with addon is unwanted behaviour for a user and it’s better try to avoid it but if it is not possible to fix it I recommend at least open tab as a normal tab, not pinned one.