When I open and use Firefox, the File, Edit, etc. strip on top of the toolbar has black lettering on a darkish grey background and is not easy to read.
However if I 'restore down' the reduced page has black lettering on a light blue background; which is far better, and I would like such a strip for the full page.
What O/S you using?
If I understand your question correctly, under Windows7 it would come under 'Themes' where you can alter colours, fonts and font sizes for windows, message boxes etc. But all the O/S are slightly different.
Then check that your monitor is set up properly. (Most monitors are poorly set up!). Go here and check that you can see all of the numbers, from 0 through to 10, on both of the numbered lines. (If you can't, follow the instructions to adjust your monitor):
http://support.mozill...ok-blurry?s=font&as=s
I am operating on 'Vista..home basic.' and have turned-on 'ClearType' but I haven't noticed any real difference. Maybe the actual type is a little sharper but, as explained, the toolbar has a pale blue background but the File, Edit,View etc. strip has a dirty grey background, unless I 'restore-down' the page and then the strip takes on the blue background.
I still think that you need to check your monitor (as per my second link) but, as others have indicated, you can change the way that Firefox is displayed by simply switching themes.
The default Addon Theme does not have great contrast.
As above .. a different theme will be your answer.
You can switch any added theme on/off at will in Tools/Addons/Appearance tab.
Problem now sorted. I took a chance and played around in the 'Personalisation' section within the Control Panel and, changing a setting from Vista standard to Vista basics, appears to have done the trick.