You can do either or both of the two things mentioned here to stop FF from behaving like that :-
1. Disable Protected Mode in Flash :
To do it, go to one of these locations
(For Windows 32bit) >> C:\Windows\System32\Macromed\Flash
(For Windows 64bit) >> C:\Windows\SysWOW64\Macromed\Flash
open the "mms.cfg" file in Notepad & add this line: ProtectedMode=0
save it & close.
Restart your browser.
(Note: To Re-enable protected mode, then simply delete the line ProtectedMode=0 you added.)
2. You can also change a specific configuration in Firefox, if the above doesn't work.
Open a new tab in FF, type or paste about:config in the address bar and press Enter. Click the
button promising to be careful.
In the search box that appears above the list, type or paste: dom.disable_window_flip
Double-click dom.disable_window_flip to switch it between true and false.
Restart FF.