The problem seems to be that the activate batch file having issues to set the write value of the variable PYTHONINSTALL. I solved this by setting it manually and deleting all the bloated function Mozilla used to detect the path.
open the bin\activate.bat file with an editor (np++ makes it clear to work with) under the :CheckPython label delete the function and the comments and use this to set your python installation path:
:CheckPython
::CheckPython(retVal, key)
::Reads the registry at %2% and checks if a Python exists there.
::Checks both HKLM and HKCU, then checks the executable actually exists.
SET key=%2%
SET "%~1="
SET reg=reg
if defined ProgramFiles(x86) (
if exist %WINDIR%\sysnative\reg.exe SET reg=%WINDIR%\sysnative\reg.exe
)
rem here you should make sure to set the correct path
set PYTHONINSTALL=C:\Program Files\Python27
if exist %PYTHONINSTALL%\python.exe goto :EOF
if exist %PYTHONINSTALL%\PCBuild\python.exe (set "PYTHONINSTALL=%PYTHONINSTALL%\PCBuild" & goto :EOF)
if exist %PYTHONINSTALL%\PCBuild\amd64\python.exe (set "PYTHONINSTALL=%PYTHONINSTALL%\PCBuild\amd64" & goto :EOF)
GOTO :EOF