This is one for the “it’s easy once you know” category.
In recent versions of Firefox the option to turn off JavaScript had apparently disappeared from the options. Recently I found out where those sneaky Mozilla devs have hidden this handy feature.
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.
In those settings, scroll down to the Advanced Settings and flick the “Disable Javascript” check-box. Voila. JS is now disabled for your debugging purposes.
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.