This was found via try a4c7ebf73c45. Full logs can be found here.

To enable this check add the following to your mozconfig:

ac_add_options --enable-undefined-sanitizer="enum"

src/media/webrtc/trunk/webrtc/modules/desktop_capture/desktop_and_cursor_composer.cc:197:11: runtime error: load of value 3840206052, which is not a valid value for type 'MouseCursorMonitor::CursorState'
    #0 0x7f7b36a53af9 in webrtc::DesktopAndCursorComposer::OnCaptureResult(webrtc::DesktopCapturer::Result, std::unique_ptr<webrtc::DesktopFrame, std::default_delete<webrtc::DesktopFrame> >) src/media/webrtc/trunk/webrtc/modules/desktop_capture/desktop_and_cursor_composer.cc:197:11
    #1 0x7f7b36a65cb7 in webrtc::ScreenCapturerX11::CaptureFrame() src/media/webrtc/trunk/webrtc/modules/desktop_capture/linux/screen_capturer_x11.cc:177:14
    #2 0x7f7b32cef869 in webrtc::DesktopCaptureImpl::process() src/dom/media/systemservices/video_engine/desktop_capture_impl.cc:648:38
    #3 0x7f7b32cef128 in webrtc::DesktopCaptureImpl::Run(void*) src/dom/media/systemservices/video_engine/desktop_capture_impl.h:236:44
    #4 0x7f7b3698661c in rtc::PlatformThread::Run() src/media/webrtc/trunk/webrtc/rtc_base/platform_thread.cc:268:10
    #5 0x7f7b36985b0c in rtc::PlatformThread::StartThread(void*) src/media/webrtc/trunk/webrtc/rtc_base/platform_thread.cc:162:40
    #6 0x7f7b4d35a6da in start_thread (/lib/x86_64-linux-gnu/libpthread.so.0+0x76da)
    #7 0x7f7b4c33888e in clone /build/glibc-OTsEL5/glibc-2.27/misc/../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Looks like the value is used, but never set: https://searchfox.org/mozilla-central/search?q=symbol:F_%3CT_webrtc%3A%3ADesktopAndCursorComposer%3E_cursor_state_&redirect=false.

It looks like the overload of OnMouseCursorPosition which actually sets the
cursor_state_ member was somehow missed during the last webrtc.org update.
This overload is present in branch 64 upstream, but has subsequently been
removed.

https://hg.mozilla.org/mozilla-central/rev/5af44ec81627