Sometimes a problem with Firefox may be a result of malware installed on your computer, that you may not be aware of.

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

• Microsoft Safety Scanner
• MalwareBytes' Anti-Malware
• Anti-Rootkit Utility - TDSSKiller
• AdwCleaner (for more info, see this alternate AdwCleaner download page)
• 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. Windows 8 has antivirus built-in already.

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

Did this fix your problems? Please report back to us!

rawellman1216 said

My 32bit Vista Acer Aspire 4730Z has recently fallen victim to the "compatibilitycheck.exe" problem. See (http://forum.kaspersky.com/lofiversion/index.php/t313939.html) for details on this problem. I repeatedly go into Safe Mode and using DOS functionality delete the offending files ,compatibilitycheck.exe and others in a directory named "Compatibility Verifier" which often have spawned into all user's account on the PC. I do this with WiFi off, ie. not connectivity to the greater web. I reboot with WiFi off in normal mode and the PC is lightening fast. However after I reconnect to the web via WiFi the infection/virus or whatever it is returns, hogs CPU, and brings my Acer to it's Knees. I can watch the spawning take place before my eyes in Windows Task Manager "Processes" where multiple instances show up each grabbing CPU. Am running FireFox 36.0 with NoScript 2.6.9.16 and TBird 31.5.0, CLAMWIN ver 098.6 AV is also installed with current Virus DB always updated. No other browsers are even on my machine except the built in IE which I never touch. Could this infection possibly be coming in through my web connections through either Mozilla application AM NOW ENTERING THIS ISSUE ON A MACBOOK PRO MacBook appears to be immune from compatibilitycheck.exe issues so whatever "Troubleshooting Information" i send in by pushing "Share Data" IS IRRELEVANT to my PC issues.

Tyler, Thanks for the suggestions, will implement them and then post a reply with the outcome. I never suspected anything from Mozilla was the root cause, simply that Firefox and or TBird have a vulnerability that Malware is exploiting. My machine stays clean, after manually removing offending files -- the ones I am aware of, as long as I don't connect to the web. Go figure ?

rawellman1216 said

My 32bit Vista Acer Aspire 4730Z has recently fallen victim to the "compatibilitycheck.exe" problem. See (http://forum.kaspersky.com/lofiversion/index.php/t313939.html) for details on this problem. I repeatedly go into Safe Mode and using DOS functionality delete the offending files ,compatibilitycheck.exe and others in a directory named "Compatibility Verifier" which often have spawned into all user's account on the PC. I do this with WiFi off, ie. not connectivity to the greater web. I reboot with WiFi off in normal mode and the PC is lightening fast. However after I reconnect to the web via WiFi the infection/virus or whatever it is returns, hogs CPU, and brings my Acer to it's Knees. I can watch the spawning take place before my eyes in Windows Task Manager "Processes" where multiple instances show up each grabbing CPU. Am running FireFox 36.0 with NoScript 2.6.9.16 and TBird 31.5.0, CLAMWIN ver 098.6 AV is also installed with current Virus DB always updated. No other browsers are even on my machine except the built in IE which I never touch. Could this infection possibly be coming in through my web connections through either Mozilla application AM NOW ENTERING THIS ISSUE ON A MACBOOK PRO MacBook appears to be immune from compatibilitycheck.exe issues so whatever "Troubleshooting Information" i send in by pushing "Share Data" IS IRRELEVANT to my PC issues.

Downloaded suggested Malware removal tool msert.exe. Ran a full scan which found and removed one Trojan and one Adware malware program. Unfortunately I inadvertently closed the report details which named the two offending programs so I am unable to list their names in this reply. Can anyone tell me where msert.exe reports are stored? If so I will list them in this forum as part of a reply to this thread.

rawellman1216 said

My 32bit Vista Acer Aspire 4730Z has recently fallen victim to the "compatibilitycheck.exe" problem. See (http://forum.kaspersky.com/lofiversion/index.php/t313939.html) for details on this problem. I repeatedly go into Safe Mode and using DOS functionality delete the offending files ,compatibilitycheck.exe and others in a directory named "Compatibility Verifier" which often have spawned into all user's account on the PC. I do this with WiFi off, ie. not connectivity to the greater web. I reboot with WiFi off in normal mode and the PC is lightening fast. However after I reconnect to the web via WiFi the infection/virus or whatever it is returns, hogs CPU, and brings my Acer to it's Knees. I can watch the spawning take place before my eyes in Windows Task Manager "Processes" where multiple instances show up each grabbing CPU. Am running FireFox 36.0 with NoScript 2.6.9.16 and TBird 31.5.0, CLAMWIN ver 098.6 AV is also installed with current Virus DB always updated. No other browsers are even on my machine except the built in IE which I never touch. Could this infection possibly be coming in through my web connections through either Mozilla application AM NOW ENTERING THIS ISSUE ON A MACBOOK PRO MacBook appears to be immune from compatibilitycheck.exe issues so whatever "Troubleshooting Information" i send in by pushing "Share Data" IS IRRELEVANT to my PC issues.

Update 1 day Later:

Acer PC running 32 bit Vista remains clean, i.e. no recurrence of the "compatibilitycheck.exe" invasive problem.

Solution was:

1. installing and running full scan using msert.exe downloaded directly from Microsoft.

2. Installation of Microsoft's "Microsoft Security Essentials" AV package.

Thank you again Tyler !