You could try these global settings:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk.

(2) In the search box above the list, type or paste cache and pause while the list is filtered

(3) Double-click the browser.cache.disk.enable preference to switch the value from true to false

(4) Double-click the browser.cache.memory.enable preference to switch the value from true to false

(5) In the search box above the list, type or paste viewer and pause while the list is filtered

(6) Double-click the browser.sessionhistory.max_total_viewers preference and edit its value from -1 to 0 (that's a zero) to disable "fast back-forward" page caching

Does that do it?

I had a similar problem but then found I was looking at the wrong version of the web site?

The only bit that hadn't been set as per steps above was the browser.sessionhistory.max_total_viewers. The browser is still caching files when it shouldn't and you shouldn't have to go into the hidden config settings when there is a checkbox is dev tools that you select which "should" turn off cache and its not doing so.