How to disable cache in Firefox?

This option was actually removed in Firefox 43 because it caused this bug.

But there are still the following options available through the about:config section:

• browser.cache.disk.enable (The disk cache is used in normal mode (non Private Browsing mode) and stores persistent data on the hard drive.)
• browser.cache.memory.enable (The memory cache is used in normal mode and in Private Browsing mode and is purged when you close Firefox.)

The first should be what you want. It is inadvisable to also disable the second one though as Firefox needs some sort of cache to function properly. Information taken from here.