Rather a difficult question to attempt to answer, as you obviously have already tried the obvious solutions.

Often it is security software or Firewalls interfering especially after a Firefox update. Deleting program files is required when doing a clean reinstall and you may wish to use a clean profile but it is probably best to ensure Profiles are backed up and retained maybe just move the whole profile rather than deleting them because the information is often useful. The profile has your Bookmarks and passwords etc.

• Windows previous file versions may help if you need to recover a profile
  http://windows.microsoft.com/en-us/windows/previous-versions-files-faq#1TC=windows-7
• Back up and restore information in Firefox profiles
• Configure firewalls so that Firefox can access the Internet
• Fix problems connecting to websites after updating Firefox

See also

• Troubleshoot and diagnose Firefox problems_5-reinstall-firefox
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox
• Firefox won't start - find solutions
• http://kb.mozillazine.org/Browser_will_not_start_up

Where are you getting Firefox from, you should use the official site for instance

• http://www.mozilla.org/en-US/firefox/all/

Presumably you will be aware that supported Firefox software on Windows is a 32 bit application. That applies even when Windows 64 bit is used, as appears to be the case with the machine you posted from. I mention that in case you have been trying 64 bit builds.

What error messages are you getting ? anything in error logs ?

• http://windows.microsoft.com/en-gb/windows/what-information-event-logs-event-viewer#1TC=windows-7

Thanks for coming back to me. I've reconfigured my firewall to make sure that firefox is allowed. I've gone into firefox properties and configured it for windows 7. I've deleted my default profile so another one can be made, I've uninstalled firefox and reinstalled the latest version from the Mozilla website. It works on re-installation, but won't then reopen once closed. I've got 2 computers using firefox and it's stopped on both of them. I'm not getting any error messages, firefox shows the spinning circle and nothing happens - it just won't open. it worked fine yesterday so I really don't know what's happened.

We've had a critical update from microsoft so I think that has caused it.

That is a possibility,if so we will expect lot more posts. I do not see that yet.

This is more for other contributors or my own confirmation that there is not a big issue with Windows updates

• I am not seeing a surge of posts about Firefox not working
  • https://support.mozilla.org/en-US/questions?product=firefox&show=all&topic=fix-problems&owner=all
  • This is not much use the topics are unused anyhow but maybe it would show a big peak if everyone on Windows 7 suddenly had Firefox problems. https://support.mozilla.org/en-US/questions/dashboard/metrics?product=firefox
• We have a site for feedback but that requires a working supported version of Firefox.
  • https://input.mozilla.org/en-US/?platform=Windows%207

If you are getting the spinning circle Firefox is opening but having problems connecting. Presumably you are posting from the problem machine but using Chrome as an alternative browser.

• see Firefox can't load websites but other browsers can

If you are sure it is not a firewall or other security software causing the issue is it a malware problem ?

• Troubleshoot Firefox issues caused by malware