Although the alternative extension, FoxyProxy, has more features such as per-url proxy server selection, it was too confusing and had too much UI friction. Also I was a previous user of ezproxy but I was looking for something better.
MM3-ProxySwitch is great because of the easy button interface and free-form configuration. You can back up just the proxy configuration with a text editor (Use FEBE for real backups, of course)! Also it can load the Internet Explorer proxy configuration (unfortunately you need to manually create an about:config key! but maybe this is for those Unix users who don't want to see "Import IE").
The bad part is the free-form configuration. It is hard to figure out what the proper syntax is. Basically it seems like you cannot have a space in the name of your proxy setting, and the noProxy is a comma-delimited list of IPs or numbers without the ports. E.g. noProxy=.internal.example.com,internal2.example.com. Finally when the IE setting is imported, is the noProxy the same format? Can MM3-PS accept noProxy=*.internal.example.com for example? I though in firefox there is no * character.