Each time I open Firefox A second (mozilla)tab opens; It's not pinned and it is not my home page.
When i keep Firefox open and open a second instance only my Google+ Homepage opens.
Forgot to post my printscreen
You can check for problems with preferences and try to rename or delete the prefs.js file and possible numbered prefs-##.js files and a possible user.js file to reset all prefs to the default values.
it helps to get rid of tabs alltogether: tools --> options --> tabs --> uncheck all but the :warn me if new tab will slow down firefox. after that tabs are gone. IZ