Mozilla Firefox 153 and later

These steps clear the cache for this browser only. Each browser on your phone keeps its own separate cache.

1. Tap the three-dot menu (⋮).
2. Tap Settings, then scroll to Delete browsing data.
3. Tick Cached images and files.
4. Tap Delete browsing data and confirm.

To have Firefox do this every time you close it, use Settings → Delete browsing data on quit.