All firmware samples were built using the Keil uVision2 8051 tools (.uV2 project files). Older versions of these tools (i.e. uVision-51) will work using the .prj files. Although it is perfectly valid to bring in the firmware example source files into the old .prj files, you may have to create your own .prj file. You can use the .uv2 files as a guide as there are some slight differences in project environment settings between the two versions of Keil.