I've never found a way to adjust brightness in any browser - IE8, IE9, Chrome and Firefox - but then I'm a bit of a dinosaur with them. There is a way on my function keys to do a lot of adjusting of brightness of the entire screen image, and that does help some with our over-bright scans, though it doesn't fix the contrast problem. I think every make of computer puts them in a different place.
I've found that sharpness, brightness and contrast can be adjusted on images transferred to
PhotoSHOP (have not tried this), Word documents and PowerPoint (my current favorite). At least in PowerPoint, I can right-click copy the entire log page jpeg image directly from the transcription link, simpler than previous efforts. Which means I can stay on the same tab as a half-done transcription.
Hope this helps.OOPS correction: does this count as a Freudian addiction symptom?