In case other developers aren't aware of two of the handiest extensions for Firefox:
- MeasureIt: This puts a small widget in the left side of your status bar; when you click on the widget, your cursor turns into a crosshair which you can then click-and-drag to get a to-the-pixel measurement of any area of the page. Perfect for the times that you're wondering just how much room you have for a graphic or table.
- ColorZilla: This puts a small widget in the left side of your status bar; when you click on the widget, your cursor turns into an eyedropper which you can then use to color sample any area of the page-- and you can then copy the color value to your clipboard in RGB or hex. Great when you're wondering what nice color some other site is using, or when you need to pick one color as the matte for a transparent GIF that's going over a gradient.