Please check your security software and firewall. Firefox has just updated and maybe it is not recognised by them.

• Fix problems connecting to websites after updating Firefox

You paper to be using Firefox 34 to post. Possibly you are temporarily using a legacy version.

For a more exhaustive list of potential problems and cures see

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

Plese post back to say how you get on and what you tried.

Actually I'm using Firefox 38.0.5, which according to windows was installed today. I checked Kaspersky Pure 3.0, which is my AV and firewall & it indicates everything is cool.

It kind of seems like it must be related to the update.

Security software is often the likely cause after an upgrade.

As a test I would try disabling all the security software and the Firewall. You do not even need to be connected to the internet to test if Firefox will start, so it is safe running without security.

If that does not help then with the security still off try

1. Starting in Firefox's safemode (e.g. Hold the shift key when clicking the icon to launch Firefox)
   • See Diagnose Firefox issues using Troubleshoot Mode
2. If still having problems next try as above but this time in Windows own safe Mode (It changed with Windows 8, not sure how you do that)

You should also look in the Windows task manager is Firefox seen to be running ? If it is running and you see no sign of screen activity try killing the Firefox process from TaskManger, and then restarting Firefox onece again.

Please post back to say what happens.

I could find no conflicts with the firewall or security software.

I didn't know what else to do, so I downloaded a fresh copy of Firefox and installed it without banishing what was already there. After that, Firefox opened right up & I still had all of my bookmarks & saved passwords. Anyway, it now works fine.

Thanks for the advice.

Please also see the details of how to correctly do a clean reinstall of Firefox:

• Troubleshoot and diagnose Firefox problems_5-reinstall-firefox
  • Details in the post lower down

(~J99)

Glad its working again. Thanks for posing back. I will mark your post as solving the question.

I will also add a comment bout the Clean Install procedure is that is better at fixing some problems than is the standard install.

Clean install (Delete the programs' files NOT the profile)

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.