Firefox has for far too long been a bloated touch like UI. The tabs are just wasted space for no reason. The internet is littered with various attempts to get control back to toolbar heights, tab width and height and so on. Rather than leaving users to guess and grope at what CSS might finally set the bit you want allow users to click on something and set color, font, height/width and so on. Themes are always incomplete, one size fits none and allow for colors the user doesn't want.