Check first your plug-ins to make sure, they are up to date:

plug-in check

if this does not solve the problem, try Firefox Safe Mode to see if the problem goes away. Safe Mode is a troubleshooting mode, which disables most add-ons.

(If you're not using it, switch to the Default theme.)

• You can open Firefox 4.0+ in Safe Mode by holding the Shift key when you open the Firefox desktop or Start menu shortcut.
• Or open the Help menu and click on the Restart with Add-ons Disabled... menu item while Firefox is running.

Once you get the pop-up, just select "'Start in Safe Mode"

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, and you need to figure out which one. Please follow the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article for that.

To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

When you figure out what's causing your issues, please let us know. It might help other users who have the same problem.

Thank you.

Note that your System Details List shows multiple Flash plugins.

1. Shockwave Flash 11.8 d800
2. Shockwave Flash 11.7 r700

You can check the Flash player installation folder for multiple Flash player plugins and remove older version(s) of the plugin (NPSWF32).

• (32 bit Windows) C:\Windows\System32\Macromed\Flash\
• (64 bit Windows) C:\Windows\SysWOW64\Macromed\Flash\

You can find the installation path of all plugins on the about:plugins page.
There is no need to toggle the plugin.expose_full_path pref to true in Firefox 22 and later, so leave this pref set to false.

• http://kb.mozillazine.org/Issues_related_to_plugins#Identifying_installed_plugins
• http://kb.mozillazine.org/about:plugins
• http://kb.mozillazine.org/about:config

Did you update your extensions to the latest versions (Video Download Helper had a recent update)?