This is one for the “it’s easy once you know” category.
In the Firefox menu, go to Tools > Web Developer > Toggle Tools, or use the CTRL-I shortcut key to activate the developer tools side-panel. Once active you see one of those typical cog-wheel icons in the upper right corner showing the settings when you click it.
The “Disable Cache” option right above it is also a handy one if you are working on a page. It beats having to hit CTRL-F5 all the time anyway.
If you want an easier way to control this, store settings per site and things like that, you’re better off installing the NoScript plugin. There’s a reason this option is hidden in the developer tools after all.