Hi,

You can try installing the latest Firefox after uninstalling the currently installed one: https://www.mozilla.org/en-US/firefox/new/

Also, in the security software (Norton, other) installed, you can try deleting all existing instances of Firefox and its related processes/files in all the different configuration modules/areas of the security software like process monitoring, sub/spawned process control, virtualization, HIPS etc., in addition to the security software's main configuration panel. Instead create new fully allowed/trusted rules for Firefox + its related processes in all the different modules/panels - a genuine/original Firefox installer as well as all the installed EXEs (Application) and DLLs (Application extension) are digitally signed by Mozilla Corporation (right-click > Properties > Digital Signatures). Even otherwise, some security software may also ask again when Firefox and/or its sub processes are started, and you may be able to allow/trust at that time. Please see this.

AV/Firewalls Configuration

Try to disable hardware acceleration in Firefox.

You can use one of these to start Firefox in Safe mode.

• https://support.mozilla.org/kb/Safe+Mode

• On Windows, hold down the Shift key while starting Firefox with a double-click on the Firefox desktop shortcut
• On Mac, hold down the Options key while starting Firefox
• Help > Restart with Add-ons Disabled

• Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available"
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

• https://hacks.mozilla.org/2010/09/hardware-acceleration/

You may have a problem with a corrupted or missing system (core) font.

You can use the System File Checker to check if font files are missing or corrupted.
Open a cmd.exe window as Administrator:
Start, click Programs, click Accessories, right-click Command Prompt, and then click Run as administrator.
After the cmd.exe prompt, type: sfc.exe /scannow
http://www.sevenforums.com/tutorials/1538-sfc-scannow-command-system-file-checker.html