They originally started with Gecko and switched to Chromium.
"There were a ton of issues using Gecko, starting with (at the time) no CDM (HTML5 DRM module) so no HD video content from the major studios, Netflix, Amazon, etc. -- Firefox had an Adobe deal but it was not transferable or transferred to any other browser that used Gecko -- and running the gamut of paper-cuts to major web incompatibilities especially on mobile, vs. WebKit-lineage engines such as Chromium/Blink."
"There were a ton of issues using Gecko, starting with (at the time) no CDM (HTML5 DRM module) so no HD video content from the major studios, Netflix, Amazon, etc. -- Firefox had an Adobe deal but it was not transferable or transferred to any other browser that used Gecko -- and running the gamut of paper-cuts to major web incompatibilities especially on mobile, vs. WebKit-lineage engines such as Chromium/Blink."
https://news.ycombinator.com/item?id=28941623