Have you tried to turn off "Hardware acceleration " ? Goto menu>option>advanced>general . Uncheck Use hardware acceleration when available. May be any of installed plugin or extension cause this . So, turn them from active to "never active " and see if the problem gone. menu>addon>plugin>choose from list and turn them off. similarly do with extensions .

Install Yescript and In the YesScript Blacklist dialog, type the URL of the site that's giving you problems in the text box. (do this after restarting firefox and then go to addons tab>extension ).

Support thread : Firefox hangs or not responding

You can check for problems with current Shockwave Flash plugin versions and try this:

• make sure you have the latest Flash version:
  http://www.adobe.com/special/products/flashplayer/fp_distribution3.html
• check for updates for your graphics drive drivers
  https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration
• disable protected mode in the Flash plugin (Flash 11.3+ on Windows Vista and later)
• disable hardware acceleration in the Flash plugin
• disable hardware acceleration in Firefox
  https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

See also:

• http://kb.mozillazine.org/Flash#Troubleshooting
• https://support.mozilla.org/kb/adobe-flash-protected-mode-firefox

Start Firefox in Safe Mode to check if one of the extensions (Firefox menu button/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox menu button/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes