You may have security software blocking the install or the change of your default browser. Also is it genuine Firefox from Mozilla or some dodgy site ? Use getfirefox.com or https://www.mozilla.org/en-US/firefox/all/ and see the help article

• How to install Firefox on Windows

The site was the original Mozilla Firefox. I have McAfee as an antivirus. And also have Internet Explorer.

I had the same issue trying to download Chrome, but now McAfee warns against using that download. I had the same warning when I first tried to download Firefox, but I told it to download anyway, and now I can't use either.

Explorer still seems ok...

I have to leave for today. Thanks for the help so far. Be back tomorrow.

In case one of your extensions is involved, could you test the page in Firefox's Safe Mode? That's a standard diagnostic tool to deactivate extensions and some advanced features of Firefox. More info: Diagnose Firefox issues using Troubleshoot Mode.

If Firefox is not running: Hold down the Shift key when launching Firefox.

If Firefox is running and you can access the menu: Either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and give Firefox the OK to restart.

Both Scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Reset).

Any difference?

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

to jscher2000

firefox looks ok in safe mode. how do I fix the root problem??

to cor-el

Here's the screenshot of the box that appears. It shows up in front of the firefox screen and is about 2 inch by 3 inch. I've tried clicking on the check box and the question mark, but nothing happens. I then have to close firefox using the task manager

You can try to disable hardware acceleration in Firefox.

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

You need to close and restart Firefox after toggling this setting.

• https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

If it works in Safe Mode and in normal mode with all extensions (Firefox/Tools > Add-ons > Extensions) disabled then try to find which extension is causing it by enabling one extension at a time until the problem reappears.

Close and restart Firefox after each change via "Firefox > Exit" (Windows: Firefox/File > Exit; Mac: Firefox > Quit Firefox; Linux: Firefox/File > Quit)

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Many thanks to all. It turned out that McAfee SiteAdvisor had been added to Internet Explorer, and maybe to Firefox as well. When I tried to upgrade Firefox, it was preventing the upgrade and any attempt to reinstall it or load Google Chrome. Apparently it is a lot more restrictive than McAfee itself. Once that was disabled, everything worked fine.

Thanks to all.