Cross-origin (or "xorigin") mode allows for mochitests to be run in a cross origin iframe. Used in conjunction with fission mode, xorigin mode can be used validate site isolation in Firefox.
When cross-origin mode is enabled, the toplevel window runs at "http://mochi.xorigin-test:8888" and individual mochitests are loaded in a cross origin iframe running at "http://mochi.test:8888".
Xorigin mode can be enabled by a flag or environment variable:
./mach mochitest --enable-xorigin-tests [path to test]