I'm using FF 39.
Your FF - does it have the display with the three line box in the upper right or does it have a menu across the top (File, Edit, View, History, Bookmarks, Tools, Help).
If you have that display you're somewhere about FF version 25ish.. If so, the Add-On menu will be in Tools.. And to check your FF version, click Help, then 'About firefox'. (doing that may download the newest version - will depend on your settings.)
If you have the newer interface with that three line box in the upper right, clicking that should give you the box I mentioned with the various option thingies - New Window, Save Page, Print, History, Find, Options, Add-ons, etc. In the border at the bottom should be the question mark..
There are three lines, but it has, New Window, New Private Window, Save Page, Print, History, Full Screen, Find, Options, Add-Ons, Developer. OK...question mark.
What was the middle thing?