just opening the FF window in the same place/condition as closed will be fine but it seems currently inconsistent
Makes sense. Like I say, Firefox DOES remember the size, but DOESN'T remember the position. Assuming you have the window maxed vertically, it can't randomly position it on that axis, only left to right. And, there, it'll just end up "wherever".
So yeah: I'd say install devilspie, try either my super-simple method or olek's more specific version, and settle on whatever works best for you. Because the important thing here is this: yes, it sucks that Firefox doesn't remember its position properly (even more annoying is that it DOES actually store it all, and "Restore Session" will also reposition it correctly: it just doesn't do that by default) - and yes, devilspie is a "bigger" solution than you "really" need. But so what? FF's behavior is stupid, and it's annoying you.
You can resolve that annoyance with very little effort, so you should. Forcing yourself to have to put up with bad behavior on a daily basis just because you "shouldn't have to" is basically just spiting yourself. Fix it, move on, and never worry about it again. 