Can you try to start Firefox in Safe Mode to see if it opens?

• Windows: Hold the SHIFT key while double clicking the Firefox icon on your Desktop
• Mac: Hold the OPTION key while double clicking Firefox in your Applications folder
• Linux: Run the firefox -safe-mode command in your Terminal/Konsole (Ubuntu/Kubuntu)

If Firefox opens up fine with no problems, it's probably one of your extensions that's causing the issue. You can re-enable your add-ons one by one until you find the one that causes the issue upon being re-enabled.

Create a new profile as a test to check if your current profile is causing the problems.

Do a clean reinstall and delete the Firefox program folder before (re)installing a fresh copy of the current Firefox release.

Download a fresh Firefox copy and save the file to the desktop.

• Firefox 29.0: http://www.mozilla.org/en-US/firefox/all.html

Uninstall your current Firefox version, if possible, to cleanup the Windows registry and settings in security software.

  Do NOT remove personal data when you uninstall your current Firefox version, because all profile folders will be removed and you lose personal data like bookmarks and passwords from profiles of other Firefox versions. 

Remove the Firefox program folder before installing that newly downloaded copy of the Firefox installer.

• (32 bit Windows) "C:\Program Files\Mozilla Firefox\"
• (64 bit Windows) "C:\Program Files (x86)\Mozilla Firefox\"

  It is important to delete the Firefox program folder to remove all the files and make sure that there are no problems with files that were leftover after uninstalling.

• http://kb.mozillazine.org/Uninstalling_Firefox

Your bookmarks and other personal data are stored in the Firefox profile folder and won't be affected by an uninstall and (re)install, but make sure that "remove personal data" is NOT selected when you uninstall Firefox.

If you keep having problems then also create a new profile.

• http://kb.mozillazine.org/Profile_folder_-_Firefox
• http://kb.mozillazine.org/Profile_backup

• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Clean_reinstall

Do a malware check with some malware scanning programs on the Windows computer. Please scan with all programs because each program detects different malware.

Make sure that you update each program to get the latest version of their databases before doing a scan.

• Malwarebytes' Anti-Malware:
• http://www.malwarebytes.org/mbam.php
• SuperAntispyware:
• http://www.superantispyware.com/
• Microsoft Safety Scanner:
• http://www.microsoft.com/security/scanner/en-us/default.aspx
• Windows Defender: Home Page:
• http://www.microsoft.com/windows/products/winfamily/defender/default.mspx
• Spybot Search & Destroy:
• http://www.safer-networking.org/en/index.html
• AdwCleaner:
• http://www.bleepingcomputer.com/download/adwcleaner/
• http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml
• Kasperky Free Security Scan:
• http://www.kaspersky.com/security-scan

You can also do a check for a rootkit infection with TDSSKiller.

  Anti-rootkit utility TDSSKiller:

• http://support.kaspersky.com/5350?el=88446

See also:

  "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked

It is possible that your security software (firewall, anti-virus) blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program.

Remove all rules for Firefox and the plugin-container from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox and the plugin-container process and the updater process.

See:

• https://support.mozilla.org/kb/Server+not+found
• https://support.mozilla.org/kb/Firewalls

• https://support.mozilla.org/kb/fix-problems-connecting-websites-after-updating

Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen) as a test to see if that helps.

It will not start in "safe" mode. I get a pop-up box that says, "Checking compatibility of Add-ons". While the progress bar is all the way to the right it will not stop. When I press control/alt/delete it shows Firefox and Thunderbird running as 32 bit. Help.

The reply by Swarnava is way beyond my capability. Is there an easy way to go back to a previous version of Firefox?

Hi LoonTime,

I can give you the link to an old version of Firefox since I would be recommending an old version that will be outdated in a couple of months and will have security holes in them. Feel free to use the old unsupported and insecure versions of Firefox. Please however remember that puts your System and personal data at an increased risk. You may then wish to mitigate that by having well rehearsed bare metal restore strategies or using a virtual machine.

• See also Install an older version of Firefox

Basically, Swarnava's reply is just a bunch of troubleshooting steps to try only he didn't separate them, he just added them one-after-another.

But before I give you the link to it, can you please try a couple more steps?

It's possible your Firefox program files have become corrupted somehow, you can replace them WITHOUT uninstalling Firefox. You just hide the existing program folder and re-run the installer.

(1) Download a fresh installer from https://www.mozilla.org/firefox/all/ to a convenient location. (Scroll down to your preferred language.)

(2) Exit out of Firefox.

(3) Rename the folder

C:\Program Files (x86)\Mozilla Firefox

to

C:\Program Files (x86)\OldFirefox

(4) Run the installer you downloaded in #1. It should automatically connect to your existing settings.

I do not have the option to rename the folder, ie. that option is not highlighted.

Sorry. Pasted wrong directions. Clippings is unorganized right now for me.

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!

The same thing is happening. after the reinstall steps the window that says: Firefox Update Checking your add-ons for compatibility with this version of Firefox. The bar progresses all the way over to the right-hand side. The spinning (working indicator) just keeps spinning and it says: This may take a few minutes...

Then on the bottom of this screen is:

     Back     Next     Cancel

Non of the 3 "buttons" above are highlighted thus they don't work. If I press control/alt/delete it shows the 32 bit as running--nothing about the 64 bit and if I cancel the 32 bit line it goes right back to checking add-ons. This is crazy and I'm so frustrated.

It is possible that there is a problem with the file(s) that store the extensions registry.

Delete the files extensions.* (e.g. extensions.sqlite, extensions.ini, extensions.cache) and compatibility.ini in the Firefox profile folder to reset the extensions registry.

• https://support.mozilla.org/kb/Profiles

New files will be created when required.

See "Corrupt extension files":

• http://kb.mozillazine.org/Unable_to_install_themes_or_extensions
• https://support.mozilla.org/kb/Unable+to+install+add-ons

It appears that some sort of "update" was automatically installed and everything appears to be working fine. any thanks for all of your suggestions. Yippee--problem solved.