The SVG files exported by Carbide Create (build 621) do not render in Firefox. This is because they are missing namespace information.
Currently, SVG files start like this:
<svg width="1848px" height="864px">
Starting like this allows them to render in Firefox:
<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="1848px" height="864px">