linearGradients are invisible on firefox

Here's a working example of bar charts rendered using linear gradients. Works in Chrome/Chromium, Safari (9), but not Firefox 47/48. (on a parallel note, windows doesn't seem to know how to render blockbuilder).

Start by reproducing the linked working bar-chart example in Firefox 47/48 and comparing its linear-gradient rendering with Chrome/Chromium and Safari 9. Trace the JavaScript and d3.js rendering path used by the example; done means linear gradients render visibly in Firefox without regressing the browsers where they already work.

Written by the indexing model from the issue text.