Go to the Privacy & Security configuration panel and clear all cached web content, then go to the advanced configuration panel - by entering about:config in the address bar - and change as follows:
browser.cache.disk.enable → false
browser.cache.disk.smart_size.enabled → false
browser.cache.disk_cache_ssl → false
browser.cache.offline.enable → false
browser.cache.memory.enable → true
browser.cache.memory.capacity → 250000 (value is in KB so 250000 is approximately 250 MB)
browser.privatebrowsing.forceMediaMemoryCache → true