There are two things you can try. But first, holding down the shift key starts Firefox in Safe Mode. This disables all your add-ons and some settings. If the browser works find in safe mode, then the problem could be an add-on. But first do this. Go to Mozilla.org to download the latest version of Firefox. DO NOT UPDATE. Get the full installer. After you download the installer, remove Firefox using the windows uninstaller. Then run the FF installer you just got. You will not lose much of anything. Now try FF normally. If you still have problems, then you will need to reset the browser. This will remove ALL ADD-ONS, and reset ALL SETTINGS to their default settings. If the problem is gone, then it was one of the add-ons. Re-install your add-ons only a few at a time. Close FF, and restart after each group. Good Luck.

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The Reset Firefox feature can fix many issues by restoring Firefox to its factory default state while saving your essential information.
Note: This will cause you to lose any Extensions and some Preferences.

• Open websites will not be saved in Firefox versions lower than 25.

To Reset Firefox do the following:

1. Go to Firefox > Help > Troubleshooting Information.
2. Click the "Reset Firefox" button.
3. Firefox will close and reset. After Firefox is done, it will show a window with the information that is imported. Click Finish.
4. Firefox will open with all factory defaults applied.

Further information can be found in the Refresh Firefox - reset add-ons and settings article.

Did this fix your problems? Please report back to us!

Thank you.

Does this only happen after you (re)boot the computer?

Which security software (firewall, anti-virus) do you have?
Does it help if you wait a bit longer before starting Firefox?

I uninstalled FF and ran the FF installer, but still had the same problem. I reset FF once again and everything worked fine initially, but when I fired up the computer today, FF froze once again . As for the add-ons, I didn't have any prior to resetting, so it doesn't seem to be an add-on issue. I can only ask why do the reset settings undo themselves everytime I fire up the computer?

Firefox can't makes such changes to files when you boot up the computer, only other (security) software can do this.
If it only happens after a reboot then you can make a backup copy of all files in the profile folder and compare them to see if any have been changed.

You haven't answered my question about which security software you use as some can place browser in a sandbox environment that prevents Firefox from making permanent changes to files (i.e. all changes only last temporary during the current Windows session).

Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen) as a test to see if that helps.

@cor-ell: I use Webroot SecureAnywhere and have for some time with zero problems, so why would that have all of a sudden caused some issue with Firefox? I haven't introduced any new software of any kind, so why the sudden change? I noticed yesterday after initial start-up that the Firefox browser was behaving, but today it froze again, forcing me to reset it once again. I did notice though that when it freezes there is a recurring message in the status bar that's always from a different source. It will read "transferring data from...," but the sorce always seems to be from some unknown source. This gives me the impression that it may be some issue with malware. I've run Malwarebytes Anti-Malware several times, but nothing shows up. I've also run Webroot SecureAnywhere, but again, nothing. Is there some other effective anti-malware/virus program I could use to find possible suspicious files in hiding?