Today I encountered an issue several times: (This issue didn't ever happen before updating to today's nightly.) After using Nightly for few minutes, suddenly, those tabs which are not restored/used yet during current session become about:blank, and can't be restored anymore. Pinned tabs and the foreground tab are not affected. Currently, I'm not sure whether this issue is related with Bug 1365713/Bug 629232 or not; my pref: browser.sessionstore.restore_tabs_lazily = true services.sync.prefs.sync.browser.sessionstore.restore_on_demand = true Version 57.0a1 Build ID 20170803100352 User Agent Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:57.0) Gecko/20100101 Firefox/57.0

I saw this last week.

Seems that a duplicate of bug 1388628

cleaning per dupe