It is the state where firefox doesn’t save the information about your visited sites, passwords, cache, text entered in forms and downloaded items entries are not saved.
How can I do it?
If you are using newer version of firefox. Click on “Firefox” icon at the top left corner of the screen.
Click on “Start Private Browsing”
The second method by which firefox always starts in private browsing is as follows.
Click on “Tools” and select “options“.
Select “Privacy” Tab.
Under “History – Firefox will” drop down menu select “Use custom settings for history“.
Then put a tick mark on “Always use private browsing mode“.
Now your firefox will always start on private browsing mode. Whatever sites you visit won’t be stored in history.