I recommend you use FAQ 5 and FAQ 15 rather than a 3rd party "how to" document.
As is typical with such things, not all environments and situations are covered in the one you reference. In particular, cvsup is only for i386 architectures, and won't work for you.
FAQ 15 section 3 is all about working with ports.
In general, start with a -stable ports tree (yes, firefox35 has a -stable update in it), as described in FAQ 5.3.3, then update just www/firefox35 to -current, and begin building. You may find "make install" is all you need, but you should anticipate running into problems with dependencies
If you have never used the ports tree before, start by practicing with some other, simple ports, until you get the hang of building packages.
Or wait for a -stable update to firefox35 for the version you desire.