Hello,

Certain Firefox problems can be solved by performing a Clean reinstall. This means you remove Firefox program files and then reinstall Firefox. Please follow these steps:

Note: You might want to print these steps or view them in another browser.

1. Download the latest Desktop version of Firefox from mozilla.org (or choose the download for your operating system and language from this page) and save the setup file to your computer.
2. After the download finishes, close all Firefox windows (or open the Firefox menu and click the close button ).
3. Delete the Firefox installation folder, which is located in one of these locations, by default:
   • Windows:
     • C:\Program Files\Mozilla Firefox
     • C:\Program Files (x86)\Mozilla Firefox
   • Mac: Delete Firefox from the Applications folder.
   • Linux: If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it - see Install Firefox on Linux. If you downloaded and installed the binary package from the Firefox download page, simply remove the folder firefox in your home directory.
4. Now, go ahead and reinstall Firefox:
   1. Double-click the downloaded installation file and go through the steps of the installation wizard.
   2. Once the wizard is finished, choose to directly open Firefox after clicking the Finish button.

More information about reinstalling Firefox can be found here.

WARNING: Do not use a third party uninstaller as part of this process. Doing so could permanently delete your Firefox profile data, including but not limited to, extensions, cache, cookies, bookmarks, personal settings and saved passwords. These cannot be easily recovered unless they have been backed up to an external device! See Back up and restore information in Firefox profiles.

Please report back to say if this helped you!

Thank you.

Thank you Adriel for your prompt response. Your solution worked perfectly. I was going stir-crazy trying to find a workable solution to the "runtime" problem I had never encountered previously. -_-

Hello,

I am glad to hear that your problem has been resolved. If you haven't already, please select the answer that solves the problem. This will help other users with similar problems find the solution more easily.

Thank you for contacting Mozilla Support.

Hi Adriel,

The "Mozilla cannot find runtime" problem has returned. It occurs in two situations: 1. When Firefox tries to download updates using "updater.exe" (version 36.0.0.5531), which causes my "Norton Internet Security" program to notify me with the message: "Auto-Protect has removed security risk Suspicious.Cloud.9.B" This message is shown every time "updater.exe" activates.

2. Firefox freezes and crashes after I highlight and try to copy a URL. Then I have to implement your clean reinstall procedure but that only lasts until the next attempted Firefox update, or random freeze and crash. Any suggestions?

Hello Sage,

I recommend scanning your computer first using your Antivirus Software to ensure there are no other viruses or malware is running on your computer that prevent Firefox to run well. Then try downloading Mozilla Firefox from its official site https://www.mozilla.org/firefox/new/?scene=2#download-fx

Thank you.

sage68 said

Mozilla Firefox updated to version 36.0. When I click on the Firefox icon to open it I receive the message: "Mozilla cannot find runtime" I receive the same message if I try to open Firefox by selecting "firefox.exe" from within the Program Files. I subsequently uninstall Mozilla Firefox and reinstall it. It will open and run normally right after the re-installation. Then if I close the program and try to re-open it again, I get the same "Mozilla cannot find runtime" error message. The only way I can get Firefox to open is if I uninstall and re-install it every time I want to open it after the initial re-installation.

This page is more confusing than helpful. I was notified to upgrade from Firefox 36.0.1 to 36.0.3 last night. When told to restart Firefox, it would not launch, displaying "Cannot find Moxilla runtime." Rebooting didn't help. I can't believe a browser publisher would put out an upgrade that sabotages it's own product and renders it useless, leaving the end user to uninstall-and-reinstall whatever to try to make it work. How can I just undo the upgrade and move on without it. 36.0.1 was just fine. Firefox is becoming too much trouble to maintain. Mine's a desktop w/WinXP. I'm ignoring the same upgrade that keeps popping up on my laptop WinXP.

Hi Adriel,

I've followed your instructions, but no Firefox installation folder shows up in the C:\Program Files\Mozilla Firefox folder, so I'm stuck. What am I missing? Am I supposed to run the Firefox Setup 36.0.3.exe before searching for the installation folder?

Thanks for any help.

Note that Firefox 36.0.4 has been released, so you can download that version or update if you have installed Firefox 36.0.3

Try this:

• Download the full Firefox installer and save the file to the desktop
  https://www.mozilla.org/en-US/firefox/all/
• Start the installation with a double-click on the desktop icon

You only need to remove the Firefox installation folder if there are problems.

sage68 said

Hi Adriel, The "Mozilla cannot find runtime" problem has returned. It occurs in two situations: 1. When Firefox tries to download updates using "updater.exe" (version 36.0.0.5531), which causes my "Norton Internet Security" program to notify me with the message: "Auto-Protect has removed security risk Suspicious.Cloud.9.B" This message is shown every time "updater.exe" activates.

I also have experienced this during several upgrades, not just the latest. (I am using Windows XP and Norton Security Suite.) Norton identifies c:\program files\mozilla firefox\updated\xul.dll with the Suspicious.Cloud.9.B virus and removes the file. I can set the Norton installation to accept xul.dll but it would make more sense for a Mozilla expert to contact Norton and work out a permanent solution, for all who are facing this problem.