At the top of the Firefox window, click the Firefox button, go over to the Help menu and select About Firefox.(For WinXP: go to the Help menu and select About Firefox)
The About Mozilla Firefox window will appear. Your version number is listed underneath the Firefox name.
your useragent said that you have 15.0.1 (User Agent Mozilla/5.0 (Windows NT 6.1; WOW64; rv:15.0) Gecko/20100101 Firefox/15.0.1) if you want it for this computer.
it is in the box in the right called: System Details, click "More system details" and you can see "Additional System Details" with your User agent inside and if you provide other details, like, Installed Plug-ins.