Has it happened when you started the ff or when you re installing ff or after updating ff? If during starting ff it has happened then remove it with all its remaining files and again install it from the official site. And this same goes for the problem after updates and if it was during re-installing then try downloading a previous version of it and make sure ur antivirus see it as a trusted app.

Download Firefox Full Installer For All languages And Systems {web link} Save the file. Then;

Using your file browser, open the Programs folder on your computer.

Windows: C:\Program Files C:\Program Files (x86) Mac: Open the "Applications" folder. Linux: Check your user manual.

Look for, and rename any Mozilla and Firefox folders by adding .old to them.

Now run the full installer. If all goes well, remove the OLD folders when you are done.

Thanks for your reply. I did uninstall it, both using a program (Your Uninstaller) and manually, and I rebooted it and still did not work. I also installed an older version, it worked for some minutes and then in popped up the same message mentioned earlier.

Supreme Eagle. The messaged popped up right after I turned the computer on. I had updated FF days before.

FredMcD I also did what you recommended to me and it did not work. I added .old to the folders, and nothing happened

Note that some of those mozilla folders is your Profiles so do not delete them unless you do no care about your Bookmarks, Passwords and other settings.

I wonder if your Firefox is running in a Compatibility mode.

Hi James. How can I know if FF is running in a Compatibility mode. When I open the browser, immediately popped up the message and I delete it and appears again.

Open the Firefox programs folder;

Open the file browser / explorer on your computer. Note: You may have to enable Show Hidden Folders / Files. Enter this in the address bar;

Windows: C:\Program Files C:\Program Files (x86) Mac: Open the "Applications" folder. Linux: Check your user manual.

Windows Instruction; Locate Firefox.exe, right click and select Properties. Select Compatibility. See if anything is turned on. Take a screen shot if you want.

Edited wrong information

Hi Fred There is nothing selected.

Please expand. Did you find Firefox.exe <programs folder>/Firefox/Firefox.exe

What happen with the right click?

Make sure that you do not run Firefox in compatibility mode. You can open the Properties of the Firefox desktop shortcut via the right-click context menu and check the "Compatibility" tab. Make sure that all items are deselected in the "Compatibility" tab of the Properties window.

Fred and Cor-el, I did find firefox.exe and checked the compatibility tad, and all the items were deselected.

Did you do any OS or software updates about the same time or before this started happening?

No, that I recall.

Please, help. I hate using IE ;(

Did you also check any desktop shortcuts that you use to start Firefox?

Do a malware check with several malware scanning programs on the Windows computer. Please scan with all programs because each program detects different malware. All these programs have free versions.

Make sure that you update each program to get the latest version of their databases before doing a scan.

• Malwarebytes' Anti-Malware:
  http://www.malwarebytes.org/mbam.php
• AdwCleaner:
  http://www.bleepingcomputer.com/download/adwcleaner/
  http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml
• SuperAntispyware:
  http://www.superantispyware.com/
• Microsoft Safety Scanner:
  http://www.microsoft.com/security/scanner/en-us/default.aspx
• Windows Defender:
  http://windows.microsoft.com/en-us/windows/using-defender
• Spybot Search & Destroy:
  http://www.safer-networking.org/en/index.html
• Kasperky Free Security Scan:
  http://www.kaspersky.com/security-scan

You can also do a check for a rootkit infection with TDSSKiller.

• Anti-rootkit utility TDSSKiller:
  http://support.kaspersky.com/5350?el=88446

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked
• https://support.mozilla.org/kb/troubleshoot-firefox-issues-caused-malware

Before you mentioned it, I had run Malwarebytes Anti-Malware (Premium) y Bitdefender and right now I am scanning the computer using Spybot. And so far, it hasn't detected any virus

By the way, thank you so much to all of you

I scanned the computer using Malwarebytes Pro, Spybot, Kapersky and BitDefender and nothing. Still, it is not working at all Firefox.

There are two things I would like you to try.

Start Firefox in Safe Mode {web link} While you are in safe mode;

Type about:preferences#advanced<Enter> in the address bar.

Under Advanced, Select General. Look for and turn off Use Hardware Acceleration.

Poke around safe web sites. Are there any problems?

Start your Computer in safe mode. Then start Firefox. Try Safe web sites.

Starting The Computer In Safe Mode;
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