Hi. If you have to restart systems without option to restart later like when you install programs or Windows update ,you may browsing and you want to keep you tabs and windows. You may restart it without closing Firefox ,but it will cause major lag ,so I will show you how to prevent that and save your tabs to be ready after your restart finnished.
1. Go to Run > Taskmgr > ok.
2. After Task Manager opens ,go to "Processes" tab > Find process call "firefox.exe" > Click it and click "End Process".
3. A window will appear, click "End Process".
4.Close Task Manager. You can restart your computer now.
5. After restart, open firefox, and it will show a page name "about:sessionrestore".
6. Click "Restore" and every windows will come back.
Thank you! Hope this help you!