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The easiest way of emptying browser cache, no matter which Firefox version you are using, is by pressing the shortcut Ctrl+Shift+Del which (on mac use Cmd+Shift+Delete) which opens the window below:
Select in the Time range to clear menu the period that you want to delete from the history, cache etc. The default Everything is the recommended option.
In Details, you can select separately what to delete in case you do not want to delete everyting.
By clicking on Clear Now all selected items are deleted.
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