I've had a little play under the hood using

about:config

Searched for anything with the word 'cache' and modified
browser.cache.memory capacity from 40mb to 100mb
also
media.cache.size now up to 100mb

Can't say that either of these have helped

I also set browser.cache.disk.enable from false to true - but this didn't really help. In fact I tried iplayer and it went back to audio only, no video, so I reverted this setting back to false.

I hope this inspires someone who can stumble upon a fix via settings...