If you run the windows debug builds you will often get "This application has failed to start because the application configuration is incorrect..." This is because the debug builds need the msvc80 crt debug dlls. You can see this by looking in at firefox.exe with http://www.dependencywalker.com/. To get these dlls (version 8.0.50727.762) you need Visual Studio 2005 SP1. SP1 is important, plain VS 2005 will not work.
If you download the tests package you can, for example, run the reftests as follows:
python reftest/runreftest.py --appname=firefox/firefox.exe --utility-path=bin --extra-profile-file=bin/plugins reftest/tests/layout/reftests/reftest.list