I am using the latest version of Firefox with Win. 8.1. I am going to upgrade to Windows 10 since I have the Win 10 upgrade file. I just havn't used it yet. I am not one of these users that like to have the latest and greatest. I'm a "If it's working, don't fix it" type of person. That's why my truck is a 4x4 94 Dodge Ram diesel, 5 speed.
Believe me, Win 10 is a big improvement over 8 (which to put it kindly, was not one of Microsoft’s better efforts).
Do you have Pop up blocker enabled in Firefox? If so, you might disable it to see if that might be the issue.