hello, try to press ctrl+0 ('zero') while you're on the page in order to reset the zoom level or in case this doesn't work also go through the other common troubleshooting steps at Websites look wrong or appear differently than they should.

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 like Notepad (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