Strictly speaking not a Firefox question, but let's see. Unless Firefox is the only thing that has gone too dark?

For general Brightness adjustment, any consumer monitor should have buttons (or a touch interface) to adjust its settings. Here is a Viewsonic's button array for reference: http://ww2.justanswer.com/uploads/space143/2012-01-21_055518_viewsonic.jpg.

What model and make is your monitor? This is so I could look up its settings procedure/manual.

A sudden change sounds odd - at best it's just a software setting that has changed, at worst you've blown a backlight or part of it (is the brightness/dimness evenly spread or lopsided?).

Did you try to disconnect and reconnect the monitor cable(s) or reboot the computer?

I fear I have mislead you by omission. It is a software problem, but of my making. At some point, a long time ago, I accidentally happened onto a spot where I adjusted the monitor's contrast to my delight. I have done the typical 'settings' adjustments before, but this was not that or there. Recently, I again happened on a spot where there was a contrast adjustment. I clicked on something not realizing it was a contrast adjustment (it was OK before I did that), and it went back to too dark. I did not know what I did to get to those spots before, and I cannot determine how to wander back to them now. The average person with good eye sight would not find this too dark. I do. It is really quite laborious for me, and I dearly hope someone can help me find the adjustment spot again. This affects my entire monitor reception no matter what program might be showing.

Did you check accessibility settings in Windows in case you have changed some settings?

• http://www.microsoft.com/enable/products/windows7/default.aspx

Can you attach a screenshot?

• http://en.wikipedia.org/wiki/Screenshot
• https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

• Use a compressed image type like PNG or JPG to save the screenshot
• Make sure that you do not exceed the maximum size of 1 MB