Firefox has Firebug, the best JavaScript debugger on the face of the planet. The developers came up with a subset of Firebug that works on all web browers, creatively named Firebug Lite and should be an essential part of any web developers toolkit.
Quick tip - if you're just using ImageMagick to get dimensions of images, try using image_size instead, it's written in Ruby, works well, and installs really quickly, rather than the five years it seems rmagick to compile & install.