Clear the cache

Click the menu button and select Settings.
Select the Privacy & Security panel.
In the Cookies and Site Data section, click Clear Data…
- The Clear browsing data and cookies dialog will open.
On the dropdown next to When:, choose Everything. Make sure that only Temporary cached files and pages is checkmarked.
- For more information about managing site data, see Manage local site storage settings.
Click the Clear button.
Close the Settings page. Any changes you've made will automatically be saved.

Tip: Here's another way to clear the Firefox cache:

Click the menu button to open the menu panel.
Click History and select Clear Recent History…
Next to When:, choose Everything from the drop-down menu, select Temporary cached files and pages in the items list, make sure other items you want to keep are not selected and then click the Clear button.

See this article for details.

Automatically clear the cache

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

Click the menu button and select Settings.
Select the Privacy & Security panel and go to the History section.
In the drop-down menu next to Firefox will, choose Use custom settings for history.
Select the check box for Clear history when Firefox closes.
Click the Settings… button. The Clear browsing data and cookies dialog will open.
Put a check mark next to Temporary cached files and pages and make sure other items you want to keep are not selected.
- For more information about other settings, see Delete browsing, search and download history on Firefox.
Click Save Changes to close the Clear browsing data and cookies dialog.
Close the Settings page. Any changes you've made will automatically be saved.