The exclusion is in the File System Shield settings as mentioned and is a default setting, but excluded only for Write options. This would mean, New creations of that file or modifications to that file are excluded from the file shield scan.
So avast isn't going to be impacting on that file.
I have the same tabs displayed as those that were open when I closed firefox, so it is restoring my session correctly.
EDIT: I also use the TabMix add-on for managing tabs, so that could also have an impact, like you I don't have sessionstore.js in the firefox/profile location.