Ran into this just now, and I think a few other times. Firefox closes entirely and I see the crash reporter dialog.

Maybe Fission related. (DOMFissionEnabled=1)

Crash report: https://crash-stats.mozilla.org/report/index/68257eee-ef56-439e-a1da-18e5f0210227

Reason: SIGSEGV /SEGV_ACCERR

Top 10 frames of crashing thread:

0 libc.so.6 __memcpy_sse2_unaligned_erms 
1 libxul.so scale_blit gfx/wr/swgl/src/composite.h:65
2 libxul.so Composite gfx/wr/swgl/src/composite.h:443
3 libxul.so webrender::compositor::sw_compositor::SwCompositeThread::process_job gfx/wr/webrender/src/compositor/sw_compositor.rs:673
4 libxul.so webrender::compositor::sw_compositor::SwCompositeThread::wait_for_composites gfx/wr/webrender/src/compositor/sw_compositor.rs:832
5 libxul.so <webrender::compositor::sw_compositor::SwCompositor as webrender::composite::Compositor>::end_frame gfx/wr/webrender/src/compositor/sw_compositor.rs:1757
6 libxul.so webrender::renderer::Renderer::render_impl gfx/wr/webrender/src/renderer/mod.rs:2287
7 libxul.so webrender::renderer::Renderer::render gfx/wr/webrender/src/renderer/mod.rs:1886
8 libxul.so wr_renderer_render gfx/webrender_bindings/src/bindings.rs:637
9 libxul.so mozilla::wr::RenderThread::UpdateAndRender gfx/webrender_bindings/RenderThread.cpp:482