When you enable "Clear history when Firefox closes" and check the box for "Browsing & Download History", Firefox also deletes the session history files, making it impossible for the "Open previous windows and tabs" feature to work as expected. If a user selected to clear "Browsing & Download History" to remove all traces of visited URLs, then it totally makes sense to also clear the session history files, but several users have reported that they want to clear all the history OTHER THAN the tabs in the restored session. For example, open tab URLs could be listed as a separate category:
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