Do you mean when Firefox is the active window and also fullscreen ?

if yes maybe you can use the trick I suggested on reply# 7 here

basically you don't get the fullscreen but using Sizer you can resize every window whenever you want setting few pixels free from the top (or every other edge)

more precisely: don't maximize \ full screen Firefox using the window system button or F11 or any Addon

1) resize the Firefox window manually (with your mouse) but don't reach the edges of the screen just leave 2\3 pixels free

I'm sure that ObjectDock will reappear, on restart Firefox will remember the last size window and position

2) Sizer will be useful to automize the process just in case Firefox window will change but you have to spend some minutus to set the same values you found on point 1)

btw I also have Firefox 15 ... and let me know

If you move your dock to the top of the screen until you can get this issue resolved, none of these issues occur.