Bug 1581862 - Updated disable for browser_windowProxy_transplant.js on macosx when Fission is not enabled. r=jmaher

Differential Revision: https://phabricator.services.mozilla.com/D59081

--HG--
extra : moz-landing-system : lando

dom/tests/browser/browser.ini

@@ -105,6 +105,6 @@ support-files =
 [browser_keypressTelemetry.js]
 skip-if = webrender
 [browser_windowProxy_transplant.js]
-skip-if = debug && bits == 64 && ((os == 'linux') || (os == 'mac' && os_version == '10.14'))#Bug 1582568
+skip-if = bits == 64 && ((debug && os == 'linux') || (!fission && os == 'mac')) #Bug 1582568; Bug 1581862
 support-files =
   file_postMessage_parent.html