After installing the A9 toolbar for testing, I decided I wanted to know just when they were contacting their server. I installed the Firefox versions of four toolbars and used netstat to see when they connected.

• A9 Toolbar: Constant connections to hosts at amazon.com and alexa.com, but only when the toolbar is visible.
• Google Toolbar: Opens initial connection to a Google-owned IP address. If PageRank display is enabled, or was earlier in the session, maintains continuous connections—even when the toolbar is hidden!
• Yahoo! Toolbar: Opens initial connections to a Yahoo server and to unknown.Level3.net (which, based on traceroute, appears to be on the way from here to Yahoo). Sometimes the latter remains open for a long time before closing. It does not appear to reconnect on its own.
• StumbleUpon: Only connects when you press its buttons.

Overall, these toolbars seem to behave in a privacy-friendly way. But it was disturbing that the Google toolbar keeps a connection open even when it’s hidden, and that disabling PageRank display doesn’t seem to stop the connections until you restart Firefox. (Maybe it does eventually, and I didn’t wait long enough.) If I’ve hidden the toolbar, I don’t need the functionality right then. There’s no reason to hold a network connection open until I re-show the toolbar.

If I only want to use these toolbars occasionally, I can just hide most of them through the View→Toolbars submenu. But to keep the Google Toolbar from phoning home, I have to either disable PageRank and restart Firefox, or disable the toolbar in the Extensions—and restart Firefox.