Comment by bluGill

For a browser UI 1cm is NEVER a useful measurement. 1cm is useful if you are writing CAD software (which you could write as a web app so firefox should know what 1cm is only for purposes of supporting such web apps), so you can touch a physical part to your monitor and visually verify it fits before having a one off part made to do further verification of your design. For everything else what matters isn't the physical size, but if it is visible, which in turn is a function of how far away the monitor is from your eyeballs and your glasses. My phone, laptop, desktop, and movie room (I don't have all of the above, but someone does) all need different sizes of address bar to be useful.