Update run instructions.

Author: juj

README

@@ -127,7 +127,9 @@ The Open POSIX Test Suite is not affiliated with the IEEE or The Open Group.
 
 This github repository adds Emscripten testing harness for pthreads. To run the Emscripten pthreads tests, perform the following:
 
-1. Add directory posixtestsuite/emcc/ to PATH.
-2. Navigate to posixtestsuite/conformance/interfaces/ as the current directory.
-3. Make sure to close all existing instanced of Firefox tabs and processes, because the test runner will forcibly terminate browser processes during the run.
-4. Run 'run_all_tests.py'
+1. Adjust the Firefox Nightly pref dom.workers.maxPerDomain in about:config to at least 40 to allow that many simultaneous threads to run (some tests need it)
+2. Prepare Firefox Nightly prefs to be emrun-friendly by setting http://kripken.github.io/emscripten-site/docs/compiling/Running-html-files-with-emrun.html#running-web-pages-in-firefox
+3. Add directory posixtestsuite/emcc/ to PATH.
+4. Navigate to posixtestsuite/conformance/interfaces/ as the current directory.
+5. Make sure to close all existing instanced of Firefox tabs and processes, because the test runner will forcibly terminate browser processes during the run.
+6. Run 'run_all_tests.py'