This is an oddity I have run into many of times. Open a new Window in Firefox (not tab) and load a Flash Video for the first time and suddenly Firefox jumps the focus back to the previous window. The window with the Flash Video is still open, it just in the background now. This even happens if the new window is a Private Browsing Window. Like I said, I’ve been dealing with this for a long while and just learned to deal with it. Recently though, someone posted about this at Go Firefox! My co-author, Gareon happened to find a solution at Mozilla Support.
It seems to be a glitch with Firefox and Adobe Flash’s “Protected Mode” on newer versions of Windows:
“Starting in Flash 11.3 the plugin runs in Protected Mode on Vista and above. This has been causing problems for some Firefox users.” (Our issue is however not listed on that KB page.)
By adding the line ProtectedMode=0 to “C:\Windows\SysWOW64\Macromed\Flash\mms.cfg”
I have had this issue in Windows 8 as well back when I was running Windows 7 (Flash version 11.3 was released in August 2012). This seemed like a simple fix so I went ahead and tried it. Bottom line, editing that file and adding that one line of code to file did fix my issues. However, trying to edit (more like save) the file was not so simple. Not sure about Windows 7, but in Windows 8 default security permissions on this file do not allow for editing by ANY user (including administrator). I ended up having to modify the security permissions, by giving my user permission to modify the file. Looking back, I am wondering if I could have edited/saved the file without having jumped through all those hoops had I run Notepad++ as administrator.
Nonetheless, when I went back into Firefox and opened a new window and then went to play a Flash video, Firefox did not “jump” to the previous window.