Try Firefox Safe Mode to see if the problem goes away. Safe Mode is a troubleshooting mode, which disables most add-ons.

(If you're not using it, switch to the Default theme.)

• On Windows you can open Firefox 4.0+ in Safe Mode by holding the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac you can open Firefox 4.0+ in Safe Mode by holding the option key while starting Firefox.
• On Linux you can open Firefox 4.0+ in Safe Mode by quitting Firefox and then going to your Terminal and running: firefox -safe-mode (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)
• Or open the Help menu and click on the Restart with Add-ons Disabled... menu item while Firefox is running.

Once you get the pop-up, just select "'Start in Safe Mode"

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, and you need to figure out which one. Please follow the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article for that.

To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

Please report back soon.

I got this malware ( red double underlined hyperlinks -- added to most webpages - with hover text popup to aol.careerbuilder.com) after downloading and installing MYCON from TUCOWS.com.

It installed a toolbar and extension which I immediately deleted -- but Firefox is screwing up now -- where its not completely deleting it.

Tell me how to get rid of this! I restarted Firefox and even shutdown Win7 ! There is nothing in my EXTENSIONS or PLUGINS causing this.

You can try these free programs to scan for malware, which work with your existing antivirus software:

• Microsoft Safety Scanner
• MalwareBytes' Anti-Malware
• TDSSKiller - AntiRootkit Utility
• Hitman Pro
• ESET Online Scanner

Microsoft Security Essentials is a good permanent antivirus for Windows 7/Vista/XP if you don't already have one.

Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Did this fix your problems? Please report back soon.

Personally, I recommend using Hitman Pro.

I had to get rid of the malware imported with either the AOL toolbar or some other thing I accidentally downloaded. These two steps fixed the problem:

1. Download Temp File Cleaner (TFC). (removed suspected spam link -mjb) Double click on TFC.exe to run the program. Click on Start button to begin cleaning process. TFC will close all running programs, and it may ask you to restart computer.

2. Download AdwCleaner by Xplode onto your desktop.

   Close all open programs and internet browsers.
   Double click on adwcleaner.exe to run the tool.
   Click Delete.
   Confirm each time with OK.
   Your computer will be rebooted automatically. 
   A text file will open after the restart.

I'll be running the temp directory cleanup once a week.

If the problem is fixed and malware was the issue, please mark my 2nd post as the solution.

This will help other users with a similar problem, by allowing them to see the answer that help you so that they can try it as well.

Thank you for using the Mozilla support forum.

It appears that Firefox v24 has introduced a vulnerability. - After removing a toolbar and extension that was creating double-underline links on pages -- this adware still exists. There is no remaining toolbar nor extension, showing, that could cause this , but Firefox has not totally removed the culprit.

I had to reinstall Firefox, and I'm suddenly getting the red underlined words in v. 26.0. I just went through a virus removal, and I don't want to start installing stuff that will hurt me again. I'm confused with all of the above info what I'm supposed to do now.

I just looked at the extensions, and there was some arcade thing there that I disabled and it seemed to fix the problem.

Go to CONTROL PANEL / PROGRAMS & FEATURES and sort to INSTALL DATE sequence - -there is the latest programs that got installed ..

UNINSTALL it/them !  It worked for me. 

I had no idea these programs got installed along with another.