Get border-color style value from extern stylesheet in FF

[Larax]

I don't know if this is a bug in FF or what - I can't get
'border-color' value from extern stylesheet using getComputedStyle(obj,
"").getPropertyValue("border-color").. More to say, I can't get ANY
border style.. It returns literally nothing, empty value although this
property is set in extern .css file..

Is there any way to get border-color property from extern stylesheet in
FF?

Thanks for help..