Click the menu button  and select Preferences.

In the Cookies and Site Data section, click Clear Data….

Tip: Here's another way to clear the Firefox cache:
Click the Library button  , click History and click Clear Recent History…. 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. See this article for details.

Automatically clear the cache

You can set Firefox to automatically clear the cache when Firefox closes:

1. Click the menu button  and select Preferences.

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.
   • For more information on the other preferences, see Delete browsing, search and download history on Firefox.
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.