Debugging
Always start with a debug build of the xforms and the schema-validation extensions. See Building Mozilla XForms for a .mozconfig for debug builds. You should create a new, clean profile for debugging. Then start Firefox out of the build directory:
cd obj-*/dist/firefox ./firefox -no-remote -jsconsole -P yourProfileName
The XForms extension that was built together with Firefox should be already installed.
Then open your form that causes problems and have a look on the console. Often you'll find the first starting points here or in the Error Console.
Schema validation problems
The schema-validation extension contains a logging facility that can show some internals on the schema processing. To activate this logging output, set the NSPR_LOG_MODULES
export NSPR_LOG_MODULES=schemaValidation:5
This only works on a debug build as described above.