Excellent, but... Rated 5 out of 5 stars
Really useful but the latest version is missing the best feature. I primarily use this tool to switch between 16 different proxies at work and having the button on the status bar was key to doing that quickly and easily.
To clarify for any other non-experts, simply right click on the status bar in order to find the "Customize" option at the bottom of the pop-up menu, the Proxy Selector button can be found in there.
You can move the button in the addon bar by using the standard customize of firefox.