Starting with Firefox 71, the resolved value of the
line-height CSS property will be
normal instead of an actual length if the specified value is
normal or undefined.
// This can now return `normal` instead of pixels window.getComputedStyle(element).getPropertyValue('line-height');
This change has already been made on the Nightly and early Beta channels since Firefox 69 after a discussion in the CSS Working Group, and Mozilla developers haven’t seen compatibility issue reports so far. The new behaviour matches Google Chrome. Safari will make the same change soon.