I think some users with Firefox 31 reported something similar with your combination of settings. In particular, you have:

• Start up automatically in private browsing mode
• Reject third party cookies

It used to be that in private browsing mode, third party cookies were accepted for the duration of the session and the setting was ignored. Now it might be honored.

To test whether this is the problem, you can modify your third party cookie setting in the Options dialog.

"3-bar" menu button (or Tools menu) > Options > Privacy

Change the "Firefox will: Never remember history" to "Firefox will: Use custom settings for history" to expose the individual options. Then change "Accept third party cookies: Never" to "Accept third party cookies: Always" and OK the change.

Any difference?

Another possibility is a proxy settings issue. You can check here:

"3-bar" menu button (or Tools menu) > Options > Advanced > Network mini-tab > "Settings" button

The default setting is "Use system proxy settings" but you could try "No proxy" to see whether that works better.

Note that your System Details list shows that you have a user.js file in the profile folder to initialize prefs each time Firefox starts.

The user.js file is only present if you or other software has created this file and normally it wouldn't be there. You can check its content with a plain text editor (right-click: Open with) if you didn't create this file yourself.

The user.js file is read each time Firefox is started and initializes preferences to the value specified in this file, so preferences set via user.js can only be changed temporarily for the current session.

• http://kb.mozillazine.org/Preferences_not_saved

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

• Switch to the DEFAULT theme: Firefox/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