Two security related updates were released yesterday, Firefox 6.0.1 and 3.6.21.

Do you recall which version you were using before?

I'm sorry, no, I don't recall. I take frequent Firefox updates and don't pay a lot of attention to the details. It's never been a problem before.

Did you see my second question? I'm having a similar problem (can't open or run) with many applications on my system, just since taking the purported Firefox update.

Well, there have been updates to both the older and the newer versions at the same time frequently as of late - and on WinXP the GUI didn't changes in 4.0 like it did on Win7 and Vista.

As far as your 2nd question, a failed or faulty update of either version of Firefox shouldn't have any effect on other installed programs. Have you tired restarting Windows? Or switching Logon User Accounts?

Yes, I tried restarting. No, I only have one logon account.

I guess your reply that there was a legitimate upgrade yesterday is what I needed to know from Firefox. The problem may be bigger than Firefox, but I still wish I could see my browser (Google Chrome is the solution at present, but it's really limited.)

I just checked programs at the Control Panel window. I took Firefox 6.0.1; maybe I shouldn't have?

Try the Firefox SafeMode to see how it works there.
A troubleshooting mode, which disables most Add-ons.
(If you're not using it, switch to the Default Theme.)

• You can open the Firefox 4/5/6/7 SafeMode by holding the Shft key when you use the Firefox desktop or Start menu shortcut.
• Or use the Help menu item, click on Restart with Add-ons Disabled... while Firefox is running.
  

Don't select anything right now, just use "Continue in SafeMode."

To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before using the Firefox shortcut (without the Shft key) to open it again.

If it is good in the Firefox SafeMode, your problem is probably caused by an extension, and you need to figure out which one.
http://support.mozilla.com/en-US/kb/troubleshooting+extensions+and+themes

Still nothing.

Download a fresh version of Firefox using Chrome, from here:
http://www.mozilla.com/en-US/firefox/

Then see if you can uninstall Firefox through the Control Panel - but do not select the option to Remove all Firefox data and customizations.

Install the fresh download. It should pickup your current Profile, with all your data and customizations.