Open Firefox on your computer. 
Click the menu button (three lines β°) in the top-right corner. 
Select Settings. 
In the left-hand menu, go to Privacy & Security.
Scroll down to the Cookies and Site Data section.
Click Clear Data…
You’ll see two checkboxes:
Cookies and Site Data
Cached Web Content
Click Clear.
β Done! Firefox has now removed all cache, cookies, and background data stored by websites.
Open Firefox. 
Go to the menu (β°) → Settings. 

Select Privacy & Security from the sidebar.
Scroll to the Cookies and Site Data section. 
Click Manage Data…
A list of websites that have stored data will appear.
Select the site from the list. 
Click Remove Selected, then click Save Changes. 
Confirm by clicking Remove when prompted. 
β Done! The cache, cookies, and background scripts for that specific site have been deleted.