The big endian (s390x and ppc64 in my case) platforms asserts on:
Unhandled external image format

https://searchfox.org/mozilla-central/source/gfx/webrender_bindings/RenderTextureHostSWGL.cpp#78

Apparently the imageformat mozilla::gfx::A8R8G8B8 is not supported.

Adding a naive workaroud to handle A8R8G8B8 as B8G8R8A8 in the UpdatePlanes leads not only to pinkish colours but also to eventual crash. Tabs crashes all the time, despite the content of the page is show very briefly.