Edit: I see you already tried this. Never mind!

Could you try starting up in Firefox's Safe Mode? That's a standard diagnostic tool to deactivate extensions, some advanced features, and some custom settings. More info: Diagnose Firefox issues using Troubleshoot Mode.

Hold down the Shift key when starting Firefox (i.e., hold Shift, then double-click a desktop icon or click a pinned icon on the Task Bar).

A small dialog should appear: click "Start in Safe Mode" (not Reset)

Can you get in?

Could you try replacing your Firefox program folders? Here's how:

(1) If needed, download a fresh installer for Firefox 30 from https://www.mozilla.org/firefox/all/ to a convenient location. (Scroll down to your preferred language.)

(2) Exit out of Firefox.

(3) Rename the folder

C:\Program Files (x86)\Mozilla Firefox

to

C:\Program Files (x86)\OldFirefox

(4) Run the installer you downloaded in #1. It should automatically connect to your existing settings.

Will Firefox start?

Note: Some plugins may exist only in that OldFirefox folder. If something essential is missing, look in these folders:

• C:\Program Files (x86)\OldFirefox\Plugins
• C:\Program Files (x86)\OldFirefox\browser\plugins

Will Firefox start up in the Profile Manager? Using "Run" from the Start screen that has the small application icons:

firefox.exe -P

Don't delete anything in the Profile Manager

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

If it opens, click the Create Profile button, choose a name like TEST, skip the option to change the folder location, and save the new profile. Then start Firefox in the new profile you created.

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.

Wouldn't start in Profile Manager either-- I tried that earlier. I'll try the program folder replacement and let you know how that goes. Thanks.

Renaming folder/reinstalling fixed it. Must have been corrupted somehow by the Windows update, I'm guessing. When I first started the re-installed one, it had a problem and asked to start in Safe Mode, which it did. After opening once in Safe mode, it opened normally on subsequent tries.

Thanks!

Thanks for the update. You might perform a file comparison between the new program folder and the old one to see what is different. You can use a freeware program like WinMerge for that, or whichever one you might prefer.

http://winmerge.org/

I downloaded and ran winmerge and didn't learn much. Binary files are all identical; no files new or changed in the old one in over a week ( I use it daily). The old one had Truesuite installed, but that hadn't changed since 6/21.

A mystery.  Perhaps a registry issue that got resolved when I installed the new one?

There is usually some security software or malware involved with issues like this in the Firefox program folder.