No idea if this has been posted before, but just in case it hasn't...
found this on Naturescapes...Firefox has a Add On called Histogram
Viewer...

https://addons.mozilla.org/en-us/fir...togram-viewer/

After you install it, go to an image on this site and right click on it.
A box will open up and you'll see 'Show Histogram of Image'.
Click on that and you'll see the histogram.

Of course, this assuming the image isn't right click protected.

Doug