Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the add-ons is causing the problem (switch to the DEFAULT theme: Tools > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.

In Firefox 4 Safe mode disables extensions and disables hardware acceleration.

• Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available"

You can also try to toggle some of the gfx and layers prefs on the about:config page to enable or disable some features.
Filter: /gfx|layers/

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.

Thank You cor-el !!!

Safe Mode did fix the problem. I was already using the default Theme, and none of my add-ons had any impact.

Hardware acceleration was the culprit! Perhaps something new with FF4?

You're welcome

Firefox will write directly to the screen using the Display driver and the Magnifier may not be able to read the Firefox window.

Did you try to disable only some features by toggling some prefs?

gfx.direct2d.disabled -> true
layers.acceleration.disabled -> true

No, I've not tried those. I tried your suggestions in order, and quit when one worked. I do plan on getting back to this, but probably not for a few days.

I'll keep you posted with what I find...

Well, I made it back today, but took a different path than you had suggested. Somehow my desktop appearance had changed so the Windows had transparent borders -- not just FF4.

I went to Control Panel\Personalization\Color and Appearance and unchecked the Transparency option. Then changed Color Scheme from Windows Aero to Vista Basic.

Now Vista Magnifier works with FF4 when Hardware acceleration is turned on. Not sure why my problem was only with FF4 -- perhaps because I don't use it that often (the magnifier).

Thanks again for your help.

Spent a little more time investigating those /gfx|layers/ prefs -- enough to get really confused. Seems to be related to the graphics card, which I got only to take better advantage of the monitor.

As I mentioned, losing the Aero fixed the problem AND allowed leaving Hardware Acceleration on. Aero = off also provides a much more responsive graphics experience, and is more pleasing to my eye. Not as "cool", but I'll take legible any day.

Cor-el, thanks again for your help. I'm really enjoying FF4.