Do you have the Firefox icon on your quick launch? Close Firefox. Right click on the icon and select Open New Tab/Window. Sometimes this works. If it opens, close it by Menu > File > Exit.

This is not a cure but will make it easier if Firefox locks up in Windows. https://support.mozilla.org/questions/997866 {web link}

Sorry, nope. This doesn't do anything different than what I've outline above. I tried all three (Open New Tab, Open New Window, Open Private Window) without success.

Okay, let's try this. Close Firefox. Now open your file browser, and go to the Users folder. In the search bar, enter xulstore.json. Delete All Of Them.

Now try Firefox.

I deleted the one in my USER folder with no change. There were a couple more in various C:\Mozilla\Firefox\Profiles folders, so I deleted those too, no change.

Just as an experiment, I renamed the Profile folder in my USER folder, to force Firefox to create a new one. That didn't fix the problem. It didn't create a new profile either.

I'm not certain how Firefox creates new profiles on first run. Does it build them from scratch or copy one from the Mozilla profile folder?

bdemmon said

I deleted the one in my USER folder with no change. There were a couple more in various C:\Mozilla\Firefox\Profiles folders, so I deleted those too, no change.

That's a custom location. Normally profiles are under AppData\Roaming, as I think you already know. As a shortcut:

%APPDATA%\Mozilla\Firefox\

This folder should contain a profiles.ini file (the .ini file extension may be hidden) which shows the names and paths to your profile folder(s). If you check it in a text editor like Notepad, does that file contain rational data?

If this just started after an update, there could be a problem with security software halting the startup (program control rules) or the program files might have become corrupted.

You can check your firewall and other security software for any program restrictions.

To address possible corrupted files, I suggest:

Clean Reinstall

We use this name, but it's not about removing your settings, it's about making sure the program files are clean (no inconsistent or alien code files). As described below, this process does not disturb your existing settings. Do NOT uninstall Firefox, that's not needed.

(A) Download a fresh installer for Firefox 43.0 from https://www.mozilla.org/firefox/all/ to a convenient location. (Scroll down to your preferred language.)

(B) Exit out of Firefox (if applicable).

(C) Rename the program folder as follows:

(64-bit Windows folder names)

C:\Program Files (x86)\Mozilla Firefox

to

C:\Program Files (x86)\OldFirefox

(32-bit Windows folder names)

C:\Program Files\Mozilla Firefox

to

C:\Program Files\OldFirefox

(D) Run the installer you downloaded in step (A). It should automatically connect to your existing settings.

Does a cleanly reinstalled Firefox start up normally??

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

• \OldFirefox\Plugins
• \OldFirefox\browser\plugins

Doing a "clean install" following your instructions fixed the issue. Strange, because I tried an uninstall/reinstall first and that didn't work. There must have been some refuse left over.

"Refuse happens!" Thanks for reporting back.

hmmmm....after rebooting I have the same issue again. Following the steps above fixes it, but only until I reboot again. I'm beginning to suspect some sort of malware.

You may have ad / mal-ware. Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

New computer, same issue. As I'm installing software, Firefox quits working (with same symptoms) after installing Actual Title Buttons v7. Uninstalling Actual Title Buttons fixes the issue.

Pity, I really liked ATB. But I'm not going to pay more to upgrade to version 8.