Every so often I get a link error when a module (typically mail.dll using --enable-incomplete-external-linkage) tries to link to xul.dll but is missing a new export, because dist/lib/xul.lib is the previous version. Making again in toolkit/library copies the new xul.lib correctly and I can then relink mail.dll without any problems. This is a pymake -j7 build, so there might be a race condition or some such.