Flash was chewing CPU in one of my myriad tabs, so I used Jetpack and the new AddonsManager API to whip up a quick add-on to enable and disable Flash quickly. It's <30 lines of code and an image. It puts an icon in the Firefox 4 add-on bar that toggles the plug-in, and sends a Growl notification (or whatever system your OS uses) indicating that the plug-in was successfully enabled or disabled. Screenshot:

Install FlashToggle. Requires Firefox 4 beta 7 or newer.

Clone and make your own on the Add-on Builder. This might require a minor change in the code since I built using Jetpack 0.10, which isn't released yet. Changing 'contentURL' to 'image' in the widget options should do the trick.

The entire code listing:

function toggleFlash(callback) {
  const { Cu } = require("chrome");
  Cu.import("resource://gre/modules/AddonManager.jsm", this);
  AddonManager.getAddonsByTypes(["plugin"], function(addons) {
    for (let i = 0; i < addons.length; i++) {
      if (addons[i].name == "Shockwave Flash") {
        addons[i].userDisabled = !addons[i].userDisabled;
        callback(addons[i].userDisabled);
        break;
      }
    }
  });
}

let flashLogoURL = require("self").data.url("flash-logo.jpg");
require("widget").Widget({
  label: "Toggle Flash",
  contentURL: flashLogoURL,
  onClick: function() {
    toggleFlash(function(disabled) {
      let message = "Flash is now " + (disabled ? "disabled" : "enabled") + ".";
      require("notifications").notify({
        title: message,
        iconURL: flashLogoURL
      });
    });
  }
});