Mozilla .bundle Source Files are for Mac
Mercurial Bundle format.
I've tried to open a .bundle file with 7-Zip in Windows to read the Firefox source code. Those are files for Mac.
These files are also much bigger than the .tar.xz ones.
I also tried to find a program to open them in Windows, but couldn't so far.
It seems to have the header "HG10BZ". Apparently compressed with the Python 'bz2' module.