This appears to only happen when you have both NoScript and FoxyProxy installed.
Note: When you install each by itself, this bug doesn't exist. Once you install both Add-Ons, this bug appears.
but i don't use NoScript.
@gkoppen: I'm running FF11.0 on Mint-Linux (3.0.0-16-generic #28-Ubuntu x86_64). I'm just using a few add-on's (Addblock Plus, Chatzilla, Firebug withFirecookie, FoxyProxy, XMarks), and the bug appears as soon as FoxyProxy 3.6 is enabled. I'm using FoxyProxy with some Patterns to route a bunch of sites via a vpn tunnel to an internal test stage.