It's free and lets you easily manage a proxy list from Firefox, but you'll need to install another program to use everything that ProxySel has to offer.
ProxySel fits snugly into your Firefox menu bar and features a drop-down menu for editing, sorting, and importing proxy lists. There is also a feature for easily adjusting your browser anonymity settings. Your proxy list also is displayed in a drop-down menu. Each proxy has a star-based rating system, with one star being OK, and four stars being superior. You can add your own rating to each proxy server. Beyond that, there is a button for turning the proxy on and off, which you'll see reflected in the tiny status bar shortcut. Importing a list is a simple process, and entries are quickly available from the menu. One click and the selected proxy was activated.
Our only complaint is that one of ProxySel's features, the TOR button, required a separate download from another Web site. But without that feature, it's still a very capable extension. If you're comfortable using proxy servers, this is a good program to have on-hand.