The suspense is killing me -- what is the message you're getting?

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 ++++++++++++++++++++++++++++ Look for, and remove any Mozilla or Firefox program folders. Do not remove the Mozilla Thunderbird folder if there is one.

Do Not remove the profiles folder.

After rebooting the computer, run a registry scanner if you have one. Then run the installer. +++++++++++++++++++++++++++ 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

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Thanks a lot. This was a real life saver.

Hi CrowS, could you check the Windows Task Manager? To call that up, you can press Ctrl+Shift+Esc.

The Task Manager has many tabs, probably the most relevant is called Processes. Firefox may be listed either as Firefox or as firefox.exe (the name of its program file). If you find that listed on Processes, try right-click > End Process to "kill" it.

Does that help?

If that does not help, the message may indicate there is a lock on critical files and that's why Firefox thinks it must already be running. But to keep this post short, we can come back to that issue.

Search for Task Manager then end the Program or Reboot should also end the program. Then follow Fried's instructions. Or possible : Copy out your Profile and Save.

• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data Install over top.

Reinstall with Current Release Firefox 59.0 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.