To clear your cache in Firefox 

1. Click on the menu button  to open the menu panel.
2. Click History and select Clear Recent History….
3. Next to Time range to clear, choose Everything from the drop-down menu, select Cache in the items list, make sure other items you want to keep are not selected and then click the OK button.

Tell Firefox to always clear the cache (optional)

You can also set Firefox to automatically clear the cache when Firefox closes at the end of each session.

1. Click the menu button  and select Settings.
2. Select the Privacy & Security panel and go to the History section.

3. In the drop-down menu next to Firefox will, choose Use custom settings for history.

4. Select the check box for Clear history when Firefox closes.
5. Beside Clear history when Firefox closes, click the Settings button. The Settings for Clearing History window will open.
6. In the Settings for Clearing History window, put a check mark next to Cache.

7. Click OK to close the Settings for Clearing History window.
8. Close the about:preferences page. Any changes you've made will automatically be saved.

In Firefox, incognito mode is called a private window.  To open a private window or tab in Firefox:

1. Click on the Firefox Menu in the upper-right-hand corner of the browser.
2. Select New Private Window.