Taskbar icon

In Firefox 4 you may need to show the menubar. Do View->Toolbars->Menu Bar. If you can't see the View menu, do Alt-V.

The taskbar icon is shown on the right hand side of the taskbar, which is at the bottom of your browser. It looks like this:

When the icon is blue, the plugin is enabled. You can temporarily disable the plugin by clicking once on the icon, which will turn it grey:

When the plugin is running in debug mode the icon is red:

You can also right-click on the taskbar icon to get this menu.

This allows you to.

Enable or disable the plugin. If you do this while a page is loading, then it aborts the load.
Enable or disable autosend. This is an override. So this doesn't affect what you have configured for specific messages, it's just that when you disable it here it won't autosend anything at all. When you re-enable it, it'll go back to either autosending or not depending on what you have configured for that message. You can use this when there's a message which you normally autosend, but for some reason this time want to edit before you send it.
Enable or disable crosspost check. This is an override. So if you disable crosspost checking then you won't see the crosspost or "active in" information. Obviously you need to have configured Checking for crossposts - otherwise altering this won't make any difference. This will make page loading a little faster.
Enable or disable Content Preview. This is an override. So if you disable it here, you won't see content previewed - no matter what you've configured on the Features tab. If you enable it here, then it will use what you've configured there. This will make page loading a little faster.
Turn debug trace on or off. The location of the debug file is configured from the Debug Options button you can find on the Features tab (see below).
Configure the plugin. This is a bit more convenient than going via Tools->Add-ons. This will open the Modplugin Preferences window, which will look something like this:

Work with Configuration Sets, which allow you to have multiple configurations in the same plugin.

If you don't like the taskbar icon, then you can disable it from Features->Advanced:

What is it? It's an Iznik tile. There's a larger one here.