In case you are using "Clear history when Firefox closes":

• do not clear the Browsing History

• Tools > Options > Privacy > Firefox will: "Use custom settings for history": [X] "Clear history when Firefox closes" > Settings
• https://support.mozilla.org/kb/Clear+Recent+History

Note that clearing "Site Preferences" clears all exceptions for cookies, images, pop-up windows, software installation, and passwords.

Also make sure that Session Restore is enabled and working.

You can check the browser.sessionstore.* prefs on the about:config page and reset user set (bold) browser.sessionstore.*prefs to the default value via the right-click context menu to make sure that Session Restore is enabled and working properly.

• http://kb.mozillazine.org/Session_Restore
• https://support.mozilla.org/kb/restore-previous-session

• http://kb.mozillazine.org/about:config

Thank you for your answer. Now I think I find the problem and solved it partly. But I need your help to solve it totally.

I never set the "clear the Browsing History " before, and when I checked it, it setted by the first one-"remember history". Although I changed it to third one-"use custem setting ", and [X] "Clear history when Firefox closes" , when I restart the brower , it setted to the first one again. Then I set it to third one, and [V] "Clear history when Firefox closes", setted the "clear history setting" all unable,and restart. The problem solved, and the setting also be saved to my custom setting. I think it is the problem, but I don't know why it happend.Could you show me the answer?

By the way, I followed your message , but I can't find browser.sessionstore.* prefs" on the about:config page. I just find the "browser.sessionstore.resume_session_once" and setted it to ture(I don't think it's the point~).