If you’ve used Eclipse PDT and you’ve tried launching Firefox as an external browser, you may find at times that Firefox rejects multiple instances of a browser session running by challenging you with a popup.  The way around this is to

  1. launch the Eclipse Preferences, select General | Web Browser | Use external web browser.
  2. Make sure Firefox is checked
  3. Click on the Edit button
  4. For location, enter /usr/bin/open
  5. For Parameters, enter -a /Applications/Firefox.app %URL%
  6. Save and launch your Eclipse PHP debugger, and TADA!!