Access application submenus with AppleScript
There are a couple of things.

First you will need a delay between the Activation of the App (Firefox) and the click action.

If you do not,the click action will be fire way too soon for the application to pick it up.

Secondly, in my FirfFox the Zoom in Menu item is under the View menu.

So the code needs to work back from the Zoom in menu item back to the initial Menu item taking in all the nodes in between.

click menu item "Zoom In" of menu 1 of menu item "Zoom" of menu 1 of menu bar item "View" of menu bar 1

activate application "Firefox"
delay 1
tell application "System Events"
    tell process "Firefox"
        click menu item "Zoom In" of menu 1 of menu item "Zoom" of menu 1 of menu bar item "View" of menu bar 1

    end tell
end tell


Each Sub menu item must have a controlling menu

So you could even use.

click menu item "Zoom In" of it's menu of menu item "Zoom" of it's menu of menu bar item "View" of menu bar 1