Hello alstoner,

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.

Please report back to see if this helped you!

Thank you.

If you were using the Firefox Setup Stub installer, you might have better results using the full off-line installation package.

This page downloads the "Stub" installer.
http://www.mozilla.org/en-US/firefox/fx/#desktop

While the full off-line installer is available from here.
http://www.mozilla.com/en-US/firefox/all.html

Locate the correct localization version, like English(US) for the USA, and then select the correct Operation system to the right - Windows, Mac OS X, or Linux. Once Firefox is downloaded, close the browser you were using and then start the installer.
double-click the Firefox Setup 24.0.exe file for Windows OS's.

It is possible that anti-virus software is interfering.
Try to disable the real-time (live) scanning of files in your anti-virus software temporarily to see if that helps.

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