Some problems occur when your Internet security program was set to trust the previous version of Firefox, but no longer recognizes your updated version as trusted. Now how to fix the problem: To allow Firefox to connect to the Internet again;

• Make sure your Internet security software is up-to-date (i.e. you are running the latest version).
• Remove Firefox from your program's list of trusted or recognized programs. For detailed instructions, see

Configure firewalls so that Firefox can access the Internet. {web link}

Hmm Microsoft is not really supporting the regular WinXP with updates since Microsoft made it EOL on April 8, 2014.

If you are doing the hack to get updates meant for atm's and cashiers and such then that could be the issue. Perhaps you can undo the last update to see if that helps.

Chrome, Chromium and Opera dropped support of the EOL WinXP and Vista back in April 2016 so the versions up til then are rather vulnerable.

Even if you could get Firefox 52 ESR working the last major update will be 52.9.0 ESR out on June 26 apart from any minor updates for security and allowed stability fixes. The only way to use more current versions of Firefox tyhen would be to use Windows 7 or later or perhaps dual boot with a 32-bit Linux distro.

FredMcD said My Windows Control Panel Firewall was OFF so I turned it on and unclicked Mozilla Firefox (could not launch Firefox). I also have AVG Internet Security and the Internet Security Enhanced Security tab that has a hyperlink to Application Rules has Firefox listed twice 1. Friends Out (lowest) 2. Friends Out then Auto-decide (lowest). All available options are from LOW to HIGH: Block, Friends Out, Friends In/Out, Internet Out, Friends In Internet Out, All Connections. Then there is a second items for All Other Connections with: Default Rule (Public/high risk zone), Auto-Decide, Block, Ask. 1 FireFox is set to Block, 2. is set to Auto-Decide. I did not change anything there but FireFox did not launch and is listed in Task Manager and it is quivering or flashing when highlighted (to End Process). FYI, I did try to Run ENUM but it does not exist on this machine.

James said Yes I know XP support was stopped but I get updates automatically still and they get installed for whatever they do. FYI, I am not using ATM or Cashier hacks. And I know 52.9.0esr is the last FireFox version for XP. I can't get any FireFox to work now and that is my problem. I even downgraded to FireFox 45.9.0esr to see if that would work from the Support Mozilla page through Chrome. THANKS, BUT ANY OTHER SUGGESTIONS ?

Hi, hold down the Shift key when you start Firefox to start in Safe Mode. More - Diagnose Firefox issues using Troubleshoot Mode.

If it works in Safe Mode, disable any theme or extensions you have - 3-bar menu > Add-ons > Themes/Extensions and restart Firefox; then if it's OK, re-enable them one by one until you find the culprit. You may need to restart Firefox after enabling some extensions.

Just wondering if an update to AVG has rendered it incompatible with the older version of Firefox you are using. Perhaps you could disable it completely to test?

Scribe said HI, Holding down the SHIFT key when trying to launch Firefox did not go into Safe Mode. The dialog box says: "CONFIRM EXPLORE OPTION" "Choosing to Explore 58 items at once may take a long time and cause your computer to respond slowly" OK or Cancel? I cancelled it cause XP is already slow. In XP the shift is a select ALL when clicking.

YES, I recently installed AVG Internet Security on this XP Desktop because I have had it on my Laptop and I am able to get it on all equipment under one purchase of AVG. I was using Firefox 52.7.4esr and when it wouldn't run after launch I subsequently uninstalled it and downgraded to prior Firefox releases: 52.2.1esr, 52.0.2esr, and then 45.9.0esr and none of those versions ran after launch even though it shows in Task Manger as a Process.

You may be right that AVG is the culprit. I hate to lose the internet security on my desktop. Any other work-around suggestions?

As a test, disable the AVG program. Then try Firefox on this site only. Is there a problem? If so, contact AVG support.

===edit=== Sorry, if there is NO problem, contact AVG support.

FredMcD said

THAT DID IT. I will be contacting AVG for a solution to their AVG Internet Security preventing Firefox from Launching and Running !! It wasn't MS Update, it was the install of AVG Internet Security. THANK YOU THANK YOU THANK YOU

@ ahlquinn :

Would you be so kind as to mark FredMcD's post as Chosen Solution  ? (there's a 'Solved the problem' button to the right of that post)

Thank you in advance  !

Glad to help. Safe Surfing.