Which is...  very strange.  Loading the SVG directly does display
correctly in Firefox:

https://s3.amazonaws.com/assets.coveralls.io/badges/coveralls_unknown.svg

Could it somehow be something with same origin policy that makes Firefox
refuse to read the SVGs?  (I haven't actually examined the web page or
the SVGs.)

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no