Open Firefox
Click the menu button
Select Settings
Go to Privacy & Security
Scroll to Cookies and Site Data
Click Clear Data
Check Cached Web Content
Uncheck Cookies and Site Data if you only want to delete cache
Click Clear
Confirm if prompted
Open Firefox
Click the menu button
Select Settings
Go to Privacy & Security
Scroll to Cookies and Site Data
Click Clear Data
Check Cached Web Content
Uncheck Cookies and Site Data if you only want to delete cache
Click Clear
Confirm if prompted