I so f#!ing hate how "font-weight: bold" looks on mozilla (the bottom one) compare to the chrome.
Chrome looks so modern and elegant >< .
Or is this some compatibility properties that i got to add ?

@irene it is , the default font is thinner

@irene without "font-weight: bold;"

@Charon92 Maybe this one is more clear

do they use the same font?

@Charon92 In theory "bold" is equal to "700", so it should be the same on both browsers.
Unless your font doesn't have a 700 version, but for example 900, which is supported on Firefox but not supported on Chrome (which in that case tries to make bolder the 400 version in real time, and that always looks bad).