You said that a Firefox clean re-install lets Firefox start normally, BUT as soon as I restart Windows the hanging behavior returns. I'm assuming that Firefox runs normally after a clean reinstall (deleting the Firefox program folder and then reinstalling Firefox) even in Windows normal mode, as long as you don't reboot the computer.

You also said that rebooting the computer in Windows Safe Mode didn't help and Firefox will "hang" when you try to start it. This is a real puzzle. I'll escalate this question to see if anyone has a clue to what's going on.

In the meantime, even though you never mentioned that Firefox actually crashed at startup, the information that's listed in a recent crash report, if it's still available, might help. See Troubleshoot Firefox crashes (closing or quitting unexpectedly) under Get help fixing this crash and post any crash Report IDs recently submitted (they'll start with bp- )

You can also look through these articles to see if anything helps at all, while waiting for a reply:

• Troubleshoot and diagnose Firefox problems (under Troubleshoot your plugins [*] and Other solutions)
• Firefox won't start - find solutions (under No error message)
• http://kb.mozillazine.org/Browser_will_not_start_up
• http://kb.mozillazine.org/Firefox_hangs

[*] Edit: A problematic plugin can cause this issue. Ref:

• http://kb.mozillazine.org/Browser_will_not_start_up#Problematic_plugin_prevents_browser_from_starting
• /questions/958850 Firefox won't open after restart (in this case, it was the "TrueSuite Website login" plugin that caused Firefox to no longer start after rebooting the computer, after a Firefox update)

P.S. You said that the problem started with Firefox 21. You posted about it in another person's thread, back in June.

Just to see if this is an issue with Firefox 21 and above, you could try installing an earlier version, to confirm whether the problem only occurs in the later versions. If you want to try this, I would suggest uninstalling your current Firefox version, removing the Mozilla Firefox program folder and then doing a clean install of Firefox 17.0.11 esr for testing purposes. (I'm suggesting Firefox 17.0.11 esr because it's currently up-to-date as far as security updates go.)

If you would like to try that, you can download Firefox 17.0.11 esr for Windows (English-US) from here. For details on downgrading Firefox, see:

• Install an older version of Firefox
• http://kb.mozillazine.org/Go_back_to_an_old_version_of_Firefox .

Which security software (firewall, anti-virus) do you have?

If it only happens after rebooting the computer, but you can still restart Firefox after having done a clean (re)install and not having rebooted then it is possible that your security software or possibly malware is making changes that prevents further starts.

Did you try a clean install and subsequently reboot the computer in Windows Safe Mode with network support?

Also, when you do a clean reinstall:

• Are you deleting the program files after you reinstall? This is not recommended if you have to save user data. If that is the case a manual export of the user profile can be copied from the file path below in then Profile folder.

In Windows 7 these are located here:

   (32 bit Windows) "C:\Program Files\Mozilla Firefox\"

   (64 bit Windows) "C:\Program Files (x86)\Mozilla Firefox\" 

• Are you manually inserting the previous profiles to the new installation folder?

Please disable the plugin that you see in the two processes or remove it from this file: "<install directory>\plugins\"

Please let us know if removing the plugin removes the hanging.

Don't know if this will help but I recently had a problem with firefox, chrome, and ie freezing up when I attempted to do any type of search on google, yahoo, etc. I went into the properties setting by right clicking the program and choosing the compatibilities tab. Check the run as administrator option. Then click on the shortcut tab and choose the advanced button, check on the run as administrator button and agree. Now click on the apply button.This worked on firefox and chrome. On IE the only way to do this that I know of is to right click on the program and choose run as administrator each time. I also had other programs, i.e., html editor, which was freezing up as well. I set the properties to run as administrator and so far the freezing of these programs have stopped.

psuliin, I just wanted to make sure you saw the first suggestion I made, since I added this edit:

Troubleshoot and diagnose Firefox problems (under Troubleshoot your plugins [*]
<snip>
[*] Edit: A problematic plugin can cause this issue. Ref:

• http://kb.mozillazine.org/Browser_will_not_start_up#Problematic_plugin_prevents_browser_from_starting
• /questions/958850 Firefox won't open after restart (in this case, it was the "TrueSuite Website login" plugin that caused Firefox to no longer start after rebooting the computer, after a Firefox update)

If you look at /questions/958850 it's very similar to your problem so I would start by troubleshooting plugins.