This may be a number of things, but conflict is most common, so here are some suggestions that may help you get to the bottom of the matter:

Be sure that a firewall is not blocking the startup of Mozilla. Some firewalls will come with "self defense" mechanisms for spyware/malware, but these programs can also isolate useful programs as well. Norton 360, Comodo Internet Security, Zone Alarm, AVG antivirus and McAffee Internet Security Suite incorperate these methods among several others.

Basic solution: To get around this type of blocking you will have to disable the self defense mechanism in your firewall or add an exception for firefox.exe.

In depth instruction for Windows users:

'This may be hard to follow, because the posting tool does not recognize spaces, but here goes...THIS IS NOT FOR THE FAINT HEARTED.'

1)Open the mozilla browser. When nothing happens...go to next step. 2)hold CTRL, ALT and the DELETE key together. In XP you will get an immediate task manager window...Go to next step. Under Vista or Windows 7 you will get a login screen that has a START THE TASKMANAGER button at the bottom. You will push this button then go to the next step. 3)In the Windows Task Manager click the Processes tab. 4)Run down the list and find all occurrences of firefox.exe 5)right click each firefox.exe and from the drop down list choose End Process Tree then answer YES or OK to the prompt box.

Firefox is now closed...Now we can determine the issue.

6)Start firefox in safemode by clicking on START or the WINDOWS ORB to open the START MENU. 7)Choose ALL PROGRAMS or PROGRAMS 8)Find the MOZILLA FIREFOX folder and click on it 9)Choose Mozilla Firefox (Safe Mode). 10)When the prompt box appears check the boxes next to the following:

     a)Disable all add-ons
     b)Reset toolbars and Controls
     c)Reset all user prefrences to Firefox defaults

11)Click on the CONTINUE IN SAFE MODE BUTTON at the bottom.

If firefox opens, you have a plug-in that is causing your issue. If it does not then repeat steps 1-5 and then move on to step 12.

12)After all instances of firefox.exe have been ended, go to START and choose CONTROL PANEL from the start menu right hand side. XP users go to step 13...Vista users choose SWITCH TO CLASSIC VIEW on the left hand side then go to step 13...Windows 7 users choose USE LARGE ICONS from the drop down list at the upper right hand side then go to step 13. 13)Double click the WINDOWS FIREWALL icon (around the bottom of the list) and check to see if it is ENABLED. If it is not enabled, Enable it. Windows 7 and Vista users: Answer YES to enable this option ANYWAY, if you get a prompt warning of dual firewall conflicts then move on to step 14. 14)Find your security center icon in the taskbar, Usually next to the clock. Right-Click the icon and choose EXIT, DISABLE (Temporarily), or CLOSE. 15)Once you are sure that your Security Suite/Antivirus software is disabled, then Start Firefox in regular mode.

If firefox opens, you have a firewall or defense conflict and need to get your documentation to find out how to allow EXCEPTIONS to the software or internet access list. You can use firefox with these settings in the meantime, but you will not have antivirus protection. Free antivirus-only programs are available at the following locations:

NOTE: If you choose to use an antivirus only program in conjunction with windows firewall, you must remove any other antivirus programs from your computer and restart the system BEFORE you install the new antivirus software or you WILL have problems with your system SPEED and FUNCTIONALITY. Do so by going to START then choose CONTROL PANEL then choose ADD OR REMOVE PROGRAMS in XP or PROGRAMS AND SETTINGS in Vista or Windows 7 then find you program in the list. Right click it and Choose UNINSTALL or REMOVE. Be sure to RESTART the system BEFORE you choose to install the new software.

If NONE of the above has worked, then you very likely have a corrupted installation or your system may be comprimised by RootKIT or Trojan infection. Please download and complete a full scan with the tool below. REMOVE ANYTHING IT FINDS then restart the system and remove and reinstall firefox then attempt to open it again:

http://support.kaspersky.com/viruses/avptool2010?level=2

Good Luck to all. I hope that his has been helpful.

Gill Senior Tech PC VICE

I am having a similar problem. Firefox will not start, not even in safe mode. I tried the above advice, turned on Windows Firewall and turned off McAfee, but it still will not launch. I also uninstalled and reinstalled the newest firefox. I am running Vista.

Thanks for any help you can offer.

Ginger