All web browsers use what are called cookies. These contain information for the web sites. Things like preferences and log-ins are stored here. If you want, you can tell Firefox to remove / clean up things when closing.

Firefox Options > Privacy. Under History, press the button and select Use Custom Settings. Also, at the bottom under Clear History, select it and press the Settings button on the far right.

Are you closing that tab before closing Firefox or do you keep the tab open and use "Show my windows and tabs from last time" to reopen the tab automatically?

See also:

• http://kb.mozillazine.org/browser.sessionstore.privacy_level