What happens when you try to start Firefox? Does anything happen? Does the icon show on the taskbar?

Have you tried starting Firefox by opening your file browser and going to the Programs Folder?

I don't know Windows 10 Creators. Was it running? Have you tried shutting it down?

Let’s do a full clean re-install; Download Firefox For All languages And Systems {web link}

Save the file. Then Close Firefox.

Using your file browser, open the Programs Folder on your computer.

Windows: C:\Program Files C:\Program Files (x86)

Mac: Open the "Applications" folder. https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac

Linux: Check your user manual. If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it. See Install Firefox on Linux; https://support.mozilla.org/en-US/kb/install-firefox-linux

If you downloaded and installed the binary package from the Firefox download page, simply remove the folder Firefox in your home directory. http://www.mozilla.org/firefox#desktop ++++++++++++++++++++++++++++ Do Not remove the profiles folder.

Look for, and rename any Mozilla or Firefox program folders by adding .old to them. Do not rename the Mozilla Thunderbird folder if there is one.

After rebooting the computer, run a registry scanner if you have one. Then run the installer. If all goes well, remove the OLD folders when you are done. +++++++++++++++++++++++++++ If there is a problem, start your Computer in safe mode and try again.

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode This shows how to Start all Computers in Safe Mode; Free Online Encyclopedia

[SOLUTION AT THE END OF THE REPLY]

Hi FredMcD and thanks for reply.

Absolutely nothing happens when I try to start Firefox. No icon on the taskbar. Firefox process seems to start for like 1-2 seconds and then it disappears. I have tried to start it both from start menu and installation directory (firefox.exe). Also I tried to start Firefox in safe mode by holding shift key down while I was in installation directory.

Windows 10 Creators Update is the latest milestone in Windows 10. Take a quick look at https://blogs.windows.com/windowsexpe.../#u12FhusoMTCWoYCE.97 The update was finished.

I followed your steps to do a clean reinstall but it didn't help. Next I chose to reboot in safe mode. In safe mode Firefox indeed opens up. What should I do next to get it working while not being in safe mode?

EDIT: I disabled hardware acceleration, rebooted normally and Firefox still won't start. I temporarily disabled antivirus (Windows Defender) but that didn't help.

EDIT 2: I opened up Windows Event Viewer, chose Windows logs and opened Application log. When I try to start Firefox, it gives me following error (I translated this from Finnish to English): Faulty application: firefox.exe, version: 53.0.2.6333, timestamp: XXXXX (timestamp in hex) Faulty module name: INTERE~2.DLL, version: 0.0.0.0, timestamp: XXXX (again in hex)

This INTERE~2.DLL pointed to, what it seems to be, an adware called 'Interenet Optimizer' under ProgramData directory. Oddly enough Malwarebytes nor Windows Defender has ever noticed this! I am currently running AdwCleaner from Malwarebytes to see if it can get rid of the adware. But again, I had no troubles with Firefox before Creators Update, and after the update was done I was right away opening Firefox.

EDIT 3/CONCLUSION: Alright, AdwCleaner did the trick. So it was the adware which most likely got faulty during Creators Update and prevented Firefox to start up. AdwCleaner got rid of it and Firefox now starts smoothly as always.

That was very good work. Well done.