Whenever I've needed that occasionally, I've used FoxyProxy – which would fit your needs quite well. With this addon (available at Mozilla's addon site, as the link shows), you can
- automatically switch proxies based on URLs
- manually switch between your default and FoxyProxy with just a single click (de/activating FoxyProxy and use either FoxyProxy or the system default)
- switch between different proxies with just two clicks (open the dropdown, select the proxy) or a single middle-click (cycles through your proxy list, see second screenshot)

FoxyProxy: Proxy setup, selection drop-down (source: FoxyProxy; click images for larger variants)
I've been very happy with this extension, and can warmly recommend it.