This svg file, saved as "inkscape SVG"

should be rendered with an arrow head at the bottom.

See ? It hasn't any.

EDIT: Seems to be firefox-related. See below: left firefox, right chromium

I think it is due to the interpretation of 

style="stroke:context-stroke" by the browser.

It's supposed to cause the marker to inherit the stroke of the the parent object ( path , line etc ).

You can either do path>stroke on the entire path.

Or if you go into Edit>XML Editor and set the stroke to be style="stroke:black" for the <path> in the arrow marker entry.

I got this new one by doing "save as simple SVG". The file is lighter and the head stroke is displayed, but not filled.

Your're right, seems the context-stroke property not handled

[Sorry can't upload image]

<marker
       style="overflow:visible"
       id="marker1082"
       refX="0"
       refY="0"
       orient="auto-start-reverse"
       markerWidth="7.6999998"
       markerHeight="5.5999999"
       viewBox="0 0 7.7 5.6"
       preserveAspectRatio="xMidYMid">
      <path
         transform="scale(0.7)"
         d="M -2,-4 9,0 -2,4 c 2,-2.33 2,-5.66 0,-8 z"
         style="fill:context-stroke;fill-rule:evenodd;stroke:black"
         id="path1080" />
    </marker>