This bug was filed from the Socorro interface and is report bp-ee70a583-817d-48bd-85fa-ceff10181005. ============================================================= Top 10 frames of crashing thread: 0 xul.dll static void std::panicking::rust_panic_with_hook src/libstd/panicking.rs:485 1 xul.dll static void std::panicking::continue_panic_fmt src/libstd/panicking.rs:390 2 xul.dll static void std::panicking::rust_begin_panic src/libstd/panicking.rs:325 3 xul.dll static void core::panicking::panic_fmt src/libcore/panicking.rs:77 4 xul.dll static void core::option::expect_failed src/libcore/option.rs:989 5 xul.dll static struct webrender::clip_scroll_tree::SpatialNodeIndex webrender::display_list_flattener::ClipIdToIndexMapper::get_spatial_node_index gfx/webrender/src/display_list_flattener.rs:101 6 xul.dll static union core::option::Option<webrender_api::display_list::BuiltDisplayListIter> webrender::display_list_flattener::DisplayListFlattener::flatten_item gfx/webrender/src/display_list_flattener.rs:517 7 xul.dll static void webrender::display_list_flattener::DisplayListFlattener::flatten_items gfx/webrender/src/display_list_flattener.rs:312 8 xul.dll static union core::option::Option<webrender_api::display_list::BuiltDisplayListIter> webrender::display_list_flattener::DisplayListFlattener::flatten_item gfx/webrender/src/display_list_flattener.rs:638 9 xul.dll static void webrender::display_list_flattener::DisplayListFlattener::flatten_items gfx/webrender/src/display_list_flattener.rs:312 =============================================================

This is rare.