I have realized the problem before but I guess it didn't matter as much then as it did now.
What I discovered is that Firefox has a default line-height value of 1.2 for input fields which can not be changed. At least in OSX, don't have Windows so I can't confirm it there.
I did some experimenting and testing and there's just no way to change the default line-height value of Firefox. All the other browsers (ok, I just tried with Chrome and Safari) obey my value perfectly fine but not Firefox.
Has anyone ever noticed this and if yes, have you found a solution to overcome this?