Yeah that's a wordpress blog quickly put together outside of mozilla's official blogging infrastructure. There was no really good reason to do that, it was just laziness on our end.
Firefox + WebRender works great in Renderdoc, either capturing the D3D11 commands generated by ANGLE or forcing opengl. The D3D11 commands and shaders correlate pretty well with the GL commands emmitted by WebRender anyway. This tool is absolutely fantastic.
Bugs are usually quickly fixed once someone on the graphics team manages to reproduce them (in a renderdoc capture).
Unfortunately nobody has managed to reproduce it yet even though the issue has been in the team's radar for a while.
If you know that the result will not change over frames, blitting a cached image will always be faster than re-rendering.
It's not necessarily a need, just an improvement. That said pathfinder is fast enough already that it can deal with rendering interesting workloads every frames at 60fps, and will keep getting better at it, I think that there is room for improvement in the tiling phase.