Does Firefox start to load? You can check the Windows XP Task Manager (Ctrl+Shift+Esc), processes tab, for firefox.exe. It may or may not have any activity (for example, if you add columns for memory and IO).

If firefox.exe hangs in memory, you can try using End Process on it and then try starting Firefox in its Safe Mode as follows:

Hold down the Shift key when starting Firefox.

A small dialog should appear with two buttons. Click "Start in Safe Mode" button NOT the Refresh button.

Will Firefox start in its Safe Mode?

Both begin to 'start' i.e., they show up in Task Manager, albeit briefly. Then they evaporate. They do not 'hang' in memory, they do not stay around to visit; they open no windows and they take no pixels. They will not load in safe mode as they will not start up. period.

Sorry, I'm not sure what you mean by "both"?

Could you try creating a new Firefox profile. A new profile will have your system-installed plugins (e.g., Flash) and extensions (e.g., security suite toolbars), but no themes, other extensions, or other customizations. It also should have completely fresh settings databases and a fresh cache folder.

This will be in parallel to your current profile, separate folder, no data loss.

Start up in the Profile Manager using Start menu > Run, then type or paste the following and press Enter to run it:

firefox.exe -P

Does the Profile Manager dialog open?

Don't delete anything here!

Any time you want to switch profiles, exit Firefox and return to this dialog.

Click the Create Profile button, assign a name like TestSept15, and skip the option to relocate the profile folder. After creating the profile, select it and start Firefox in that profile.

Will Firefox start up?

When returning to the Profile Manager, you might be tempted to use the Delete Profile button. But... it's a bit too easy to accidentally delete your "real" profile, so I recommend resisting the temptation. If you do want to clean up later, I suggest making a backup of all your profiles first in case something were to go wrong.

running firefox.exe -P allowed me to create a new profile and Firefox to start up. By "Both" I meant either Firefox or Firefox Developer's Edition. I was having the same symptoms with either/both installation(s). Now I have to figure out what part of my profile isn't corrupted... oh joy.

What Anti-Virus security software do you have?

It is possible that your firewall or other security software blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program.

Remove all rules for Firefox and the plugin-container from the permissions list in the firewall and let your firewall ask again for permission to get full, unrestricted, access to install for Firefox and the plugin-container process and the updater process.

See:

• https://support.mozilla.org/kb/Server+not+found
• https://support.mozilla.org/kb/Firewalls

• https://support.mozilla.org/kb/fix-problems-connecting-websites-after-updating

Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen) to see if that has effect.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• http://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

Okay, you might want to reference this article: Recover important data from an old profile

Note that you should be cautious with using the same profile folder with different Firefox version, especially in case of alpha builds like the Developer Edition (a2) and the Nightly build (a1).

Note that Firefox uses two location for the profile folder (main profile and cache).

• C:\Documents and Settings\<user>\Application Data\Mozilla\Firefox\Profiles\<profile>\
• C:\Documents and Settings\<user>\Local Settings\Application Data\Mozilla\Firefox\Profiles\<profile>\