Clearing the cache in the latest Firefox versions is a relatively easy task. Everything is done within one small window.
Step 1: From the Firefox window main menu, open History > Clear Recent History.
Step 2: Select only “Cache” and the preferred time range to clear the data.
Step 3: Click “Clear Now” to free up space.
There are advanced settings and add-ons that allow you to clear the cache in Firefox automatically every time when Firefox closes, but that may slow down your experience.