This is the best way to uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files and C:\Program Files(x86) Then restart system. Then run Windows Disk Cleanup. Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved. But you should make a back up before you do : https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

Reinstall with Current Release Firefox 57.0.1 with a Full Version Installer https://www.mozilla.org/firefox/all/

Please let us know if this solved your issue or if need further assistance.

Some problems occur when your Internet security program was set to trust the previous version of Firefox, but no longer recognizes your updated version as trusted. Now how to fix the problem: To allow Firefox to connect to the Internet again;

• Make sure your Internet security software is up-to-date (i.e. you are running the latest version).
• Remove Firefox from your program's list of trusted or recognized programs. For detailed instructions, see

Configure firewalls so that Firefox can access the Internet. {web link}

FredMcD said

Some problems occur when your Internet security program was set to trust the previous version of Firefox, but no longer recognizes your updated version as trusted.

Not a communications problem, it works until I reboot. Just to be sure I disabled the firewall... no change. No window is displayed and no icon in the task bar. Firewall shows C:\Program Files\Mozilla Firefox\firefox.exe is allowed, which is the executable for 57, same as previous versions.

Pkshadow said

This is the best way to uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files and C:\Program Files(x86) Then restart system. Then run Windows Disk Cleanup. Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved. But you should make a back up before you do : https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles Reinstall with Current Release Firefox 57.0.1 with a Full Version Installer https://www.mozilla.org/firefox/all/ Please let us know if this solved your issue or if need further assistance.

Thought I was doing all that, but will re-download and make sure I didn't miss any steps. Stand by...

Many have reported issues using the 64bit Firefox. Install the 32bit version from; Download Firefox For All languages And Systems {web link}

Pkshadow said

This is the best way to uninstall Firefox. [...]

While strictly following these instructions did not fix the problem per se, it did expose what the real problem was. Isn't serendipity wonderful?

I have a program, Simple Software-Restriction Policy from IWR Consultancy, that restricts what folders programs can be run from and, in this case, what privileges they have when run. Apparently FF57 writes in folders other than the profile during launch where earlier versions did not. Unlocking (temporarily removing) the restrictions, or reconfiguring the policy to exempt FF, allows FF to launch, with the caveat that the system is not protected from downloaded malware. Ditto for v62 of Chrome.

Now the question is, what is FF57 doing that it didn't used to do that the s/w policy is blocking? I've contacted IWR in hope of them providing an update so that protection can be restored.