What's the default value?
line-height has a initial value of
normal means depends on the user agent (browser). Desktop browsers (including Firefox) use a default value of roughly 1.1~1.2, depending on the element's font-family. It's best set a line-height for consistent behavior across browsers.
What unit should be used?
Using a unitless value is the preferred way to set line-height to avoid unexpected results due to inheritance. Unitless values are multipliers against the font-size (eg. a 1.5 line-height on a 16px font-size yields a 24px line-height) and behave the same way a percentage line-height does if there is no inheritence.
How is the unitless value different from a percentage?
They're both multipliers, but behave differently when inherited.