Bug 1769241 - Skip test when fission is enabled, and run it sequentia…

…lly. r=rpl

See also: Bug 1762638

Differential Revision: https://phabricator.services.mozilla.com/D153646

toolkit/components/extensions/test/xpcshell/xpcshell-serviceworker.ini

@@ -23,5 +23,9 @@ prefs =
 [test_ext_runtime_getBackgroundPage.js]
 [test_ext_scripting_contentScripts.js]
 [test_ext_scripting_contentScripts_css.js]
+skip-if =
+  os == "linux" && debug && fission # Bug 1762638
+  os == "mac" && debug && fission # Bug 1762638
+run-sequentially = very high failure rate in parallel
 [test_ext_scripting_contentScripts_file.js]
 [test_ext_scripting_updateContentScripts.js]