Tabs Do Not Load When I Open Firefox 3.6.3

Lately, I have noticed that when I open Firefox, the tabs do not load. I opened the Options window in Firefox’s Tools menu then to Privacy > Use custom settings for history and configured some settings making sure the Automatically start Firefox in a private browsing session check box is unchecked. My settings didn’t work.

The culprits, I found out running Firefox on Safe Mode, are two of my add-ons. Disabling either the WebMail Notifier 2.4 or Torbutton 3.6.2 or both of them allows Firefox to load its tabs when opened.

Are these add-ons really the cause of tabs not loading or there is something to be configured in the about:config settings?