Hello there, have you tried uninstalling the program? If not i think you should and try restarting your pc after that.

I attempted another install and restart with no luck

jdt2313:

Are you using a tablet or computer to install Firefox? If it's a tablet, what version of Windows is it running? Go to start>type winver and let me know. If it's a computer, let me know what operating system you are using. Mac, Linux, Windows?

Thanks.

What is the name of the device you are using?

I'm running Windows 8 64 bit. I'm attempting to reinstall Firefox 27.

Have to tried reinstalling??

You said that Windows is saying it's an incompatible browser. Are you sure you're downloading from www.getfirefox.com and not another 3rd party site? I have Firefox 27 installed just fine.

please provide us with a screenshot. If you need help to create a screenshot, please see How do I create a screenshot of my problem?

Once you've done this, attach the saved screenshot file to your forum post by clicking the Browse... button below the Post your reply box. This will help us to visualize the problem.

P.S. your user agent is something else though. Mozilla/5.0 (Windows NT 6.3; Win64; x64; Trident/7.0; Touch; .NET4.0E; .NET4.0C; .NET CLR 3.5.30729; .NET CLR 2.0.50727; .NET CLR 3.0.30729; Tablet PC 2.0; InfoPath.3; MASAJS; rv:11.0) like Gecko that indicates this question was Disregard that.

I'm on a Sony Vaio T series

I'm downloading directly from Mozilla.org. After a fourth install attempt I get a class not registered message.

So you are able to open it, but you're getting that error.

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 http://www.mozilla.org and save the setup file to your computer.
2. After the download finishes, close all Firefox windows (click Exit from the Firefox or File menu).
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 run Firefox's uninstaller or use a third party remover as part of this process, because that could permanently delete your Firefox data, including but not limited to, extensions, cache, cookies, bookmarks, personal settings and saved passwords. These cannot be recovered unless they have been backed up to an external device!

Please report back to see if this helped you!

Thank you.

No luck. Still getting the same message. Firefox is not opening any windows, only the error message.

Hm,

Can you try going to that specific folder. (C:/Program Files (x86)/Mozilla Firefox) and double click on firefox.exe Does it open then?

I've tried that, the desktop shortcut, the tray icon, and clicking a link to open it as the default browser

Hello try to boot the computer in Windows Safe mode with network support (press F8 on the boot screen) and check it again.

otherwise, try a clean re-install, 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.

   Download the latest Desktop version of Firefox from http://www.mozilla.org and save the setup file to your computer.
   After the download finishes, close all Firefox windows (click Exit from the Firefox or File menu).
   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. 
   Now, go ahead and reinstall Firefox:
       Double-click the downloaded installation file and go through the steps of the installation wizard.
       Once the wizard is finished, choose to directly open Firefox after clicking the Finish button. 

Please report back to see if this helped you!

thank you

I try to avoid using Windows and rarely go near Windows 8 but afteraqick websearch maybe some of these comments help

• In this case it appears creating a new firefox profile solved the issue
  http://forums.mozillazine.org/viewtopic.php?f=23&t=2630261
  • Use the Profile Manager to create and remove Firefox profiles _creating-a-profile
• I am not suggesting you try this.
  Only note the similarity: In an issue affecting Chrome deletion of registry keys was suggested as a fix http://superuser.com/questions/503489/chrome-doesnt-start-in-windows-8-gives-an-error-message-class-not-registered.
• The generic fix to this type of error appears to be use of commandline utility Regsvr32 see
  http://www.thewindowsclub.com/register-dll-file-windows
  • http://www.thewindowsclub.com/regsvr32-in-windows-explanation-usage-and-error-messages

I will also edit the thread title

• Was
  Firefox will not start. I have tried everything listed. There are no errors. I have attempted a system restore, but nothing works.
• Now Firefox will not start. ( Class not registered )

P.S. not sure how to determine what dll is involved in firefox.exe not registered problem but it may give a start to those with Windows 8 looking at this problem.

All those with this problem can you confirm

• This is seen now on the standard Firefox for Desktops
• State whether or not you are using a touch-screen . (That may, or may not, turn out to be significant.)

I suspect this is an issue a few Windows 8 users are going to experience. This is a known issue on Firefox Windows touch /metro.

This is not something I can help with directly I do not use Windows 8. I may be mistaken but I do not think anyone is going to provide an instant solution.

As I suspect it is a Firefox/Windows issue I am going to suggest as an unofficial workaround that users with such a problem consider installing Firefox portable ESR. That is not even an official Mozilla build but

• I think it will work
• AND it (ESR) is secure. (Downgrading is not normally to be recommended the ordinary Firefox 25 and 26 are no longer supported and NOT secure)
• Portable Firefox may be installed easily and simply as an additional browser.
• See and download from http://portableapps.com/apps/internet/firefox-portable-esr

I will have to leave others to troubleshoot this issue further, and possibly consider gathering information for filing a bug.