I am running WB10 on one of my laptops, and I'm actually having a problem with Firefox SOMETIMES skinning, and other times wiping out the skin. The Firefox version is (or should be) the most recent one.
Anyway, I'm using an Aero 7 Ultimate WB skin. Sometimes I get this:
http://imgur.com/yLcPtCx
^This means that Firefox is actually skinning with my WB skin.
If I minimize to desktop or anything like that, then this happens:
http://imgur.com/1WQVops
Windowblinds then craps out the skin altogether, but can't restore the default Windows 10 skin. You have to hover your cursor in that same corner to see the minimize, maximize, and close icons, and they are in a simple small, black and white format.
I hadn't seen anyone post about having this problem, and it happens regularly to me.