Startup Open previous windows and tabs, is greyed out.

Hello, It is most likely that your history settings likely are set to "never remember". To change that, go into settings, select the privacy and security panel, and under the History section, change the drop-down to remember history. If it was originally "never remember", then a restart will be required. If not, it is most likely Firefox has loaded the previous session. to recover those tabs, go to History > Recently Closed Windows to recover lost tabs.