Does this happen only after a day or is it when you close Firefox and come back?

Make sure you are not using Firefox in Private Browsing:

https://support.mozilla.com/en-US/kb/Private+Browsing#Turn_off_Private_Browsing

Also, make sure no third party software is deleting your history overnight. Stuff like CCleaner, for example, is know to do this by default, and the user must configure it as he wants it to behave, otherwise it will clear the history.

Your plugins list shows outdated plugin(s) with known security and stability risks.

• Next Generation Java Plug-in 1.6.0_15 for Mozilla browsers

Update the Java plugin to the latest version.

• http://java.sun.com/javase/downloads/index.jsp (Java Platform: Download JRE)

• Make sure that the History is set to at least 1 day: Tools > Options > Privacy > History: "Remember visited pages for at least"
• To see History and Cookie settings in Tools > Options > Privacy, choose the setting "Firefox will: Use custom settings for history"

You can look at these prefs on the about:config page.

• http://kb.mozillazine.org/browser.history_expire_days (180) (also affects saved form data)
• http://kb.mozillazine.org/browser.history_expire_days_min (90)

Thank you so much for your helpful information.