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/.
Unfortunately these DLLs are not redistributable. To get these DLLs you only have to install the Visual C++ compilers from the Windows Vista SDK. No other packages are necessary. If you are a Mozilla employee or contractor, click for more instructions.
To run tests of 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