After pulling recent changes (I suspect the gtk2 removal), libmozwayland.so now overrides actual libwayland that's loaded by gtk3, so Wayland connections just started failing.

(lldb) bt
* thread #1, name = 'MainThread', stop reason = breakpoint 1.1
  * frame #0: 0x00003aa6c6d60084 libmozwayland.so`wl_display_connect(name=0x0000000000000000) at mozwayland.c:119:3
    frame #1: 0x00003aa6dcc6c1e3 libgdk-3.so.0`___lldb_unnamed_symbol922$$libgdk-3.so.0 + 51
    frame #2: 0x00003aa6dcc37fea libgdk-3.so.0`gdk_display_manager_open_display + 394
    frame #3: 0x00003aa6cdd15ac3 libxul.so`XREMain::XRE_mainStartup(this=0x00007fffffffd920, aExitFlag=0x00007fffffffd8a

Maybe this doesn't happen on Linux right now somehow, but really, what in the build system is supposed to ensure that mozwayland actually would be overridden at runtime?

Yep, fix incoming in bug 1706452