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….
Remove the check mark in front of Cookies and Site Data.
For more information about managing site data, see Manage local site storage settings.
With Cached Web Content check marked, click the Clear button.
Close the about:preferences page. Any changes you’ve made will automatically be saved.
Tip: Here’s another way to clear the Firefox cache:
Click on the menu button
to open the menu panel.
Click History and select 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.