Good, but width too high Rated 4 out of 5 stars
I like this add-on a lot. It's good for testing how web pages look at various resolutions.
However, I feel there's a small bug (version 1.7). With Firefox maximized, it reports the resolution in the corner at "1456x856"; this is odd, because the label on my monitor says that it has a 1440 x 900 resolution. In my Nvidia Control Panel applet, that, too, confirms that I have my resolution set at "1440 x 900 (native)." I am using a two-line permanent taskbar (Windows Vista), so I lose height pixels, but I'm puzzled how the reported width can be greater than 1440.
EDIT: After reading orev's response, below, I am now more educated, and sort of retract this bug comment... or more appropriately, perhaps I should assign it to Windows. (I'm using Vista Home Premium, SP2, by the way.)
The essence of this "bug" lies within Windows itself. When you have the browser window in "window" mode (not full screen), the height and width are calculated (by Windows) including the application area itself, and also the borders around the application. Windows 7 uses somewhat thick borders to create the appearance of glass windows. Other versions of Windows use different sizes, and it's also different on Mac and Linux.
When you maximize the window, Windows plays a trick by making the application big enough so you can't see the borders, but they are still there; off the edge of the screen. Firesizer has no way to know this, and the width of the border can be different on every system, so there is no way to simply fudge the numbers if you are in full screen mode.