In Mozilla's Firefox browser, you clear the cache from the Clear Recent History area in the browser's Options. Once there, check Cache and then tap or click Clear Now.
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The Ctrl+Shift+Del keyboard shortcut is probably the fastest way to open this tool. It's also available from Firefox's Menu button (the three-lined hamburger icon) via Options, then Privacy & Security, and finally the clear your recent history link from the History area.
See How to Clear the Cache in Firefox for a complete tutorial.
TIP: Don't forget to choose Everything from the Time range to clear set of options, assuming that's the time frame you want to clear the cache over.
If you're using Firefox's mobile app, tap the menu from the bottom right and then choose Settings from that menu. Find the PRIVACY section and tap Clear Private Data. Make sure Cache is chosen and then tap Clear Private Data. Confirm with an OK.
Firefox Focus is another mobile browser from Firefox that you can clear the cache from using the ERASE button on the top right of the app.