See Foxyproxy is already suggested, I use it - very easy to configure.
One side note. If you find you rotate your proxies and one day your browser can't log in it may be that your IP has rotated. I have DSL and my ip rotates. One day FF could not load. I opened Chrome and logged in to my proxy account, saved my IP and then everything ran fine.
Just an aside as I didn't know until it happened.