Could you delete the user.js file in your profile? This article describes how to do that: How to fix preferences that won't save. That file can switch how Firefox starts up, as well as override other kinds of preferences. Unless you created the file yourself -- in that case you might want to edit it instead of deleting it.

By the way, your More System Details shows this customized preference:

browser.sessionstore.max_resumed_crashes: 100 

Normally that value is 1 and I recommend either 1 or 0. Having a low value allows you to break out of a loop where Firefox tries to restore windows and tabs after a crash and is not successful in doing so. At that point, with the normal value of 1, Firefox does not try again and instead presents a screen listing your previous session windows and tabs so you can deselect any tabs that might be causing the problem restoring the session.

I notice that your System Details List shows multiple Flash plugins.

1. Shockwave Flash 14.0 r0
2. Shockwave Flash 13.0 r0
3. Shockwave Flash 12.0 r0

You can find the installation path of all plugins on the about:plugins page.

• http://kb.mozillazine.org/Issues_related_to_plugins#Identifying_installed_plugins

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

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