You can find TomTom’s GitHub repository at https://github.com/tomtom-international. Source code and modifications are published here.
Source code and modifications for some of the older products are published on this site.
TomTom HOME is a PC application that manages TomTom GPS devices (Portable Navigation Devices) connected to the PC. It uses Mozilla XULRunner as platform which is being shipped with TomTom HOME. TomTom made some small changes to Mozilla's source code, mostly bug fixes and branding, which are published here.
The source code changes are available under the MPL, GPL and LGPL.