In the same vein as the sticky up above for exif viewers for Firefox, here's another 'helpful' AddOn for FF to help understand the apparent brightness of an image.
many times I've seen replies to threads where the reply-ee will comment that it looks too dark or too bright, and yet on my monitor the image looks fine, and I back that up with a quick view of the histogram to confirm what I can see.
use it as you would most add ons.. just right click and select Histgram Viewer from the context menu list. Wait a few seconds and a histogram graph pops up showing you the brightness (blend of RGB) channels.. ie. overall luminance.
If you're like me and too cheap to purchase a screen calibrator (hey, I have an excuse!.. all my spare cash goes towards useless and redundant lenses )... this lil tool at least helps you understand that your monitor may be way off a reasonable calibration level.
I use this plugin all the time to verify the exposure of an image, and it saves going to the trouble of saving the image to the PC and then opening the image in an editor just to see the level of exposure.
I've checked it against many of my images both the posted version and the versions on my PC and against the histogram graph in my choice of editors(View and Capture NX's) and I've yet to see any discrepancies.
So it's fairly accurate for what it's intended use is.
been using it for months now, and it hasn't caused any issues with any of the latest versions of FF.
big from me.