It occurred to me that Huub wrote in gmane.os.netbsd.general:
> Hi,
>
> Using "lintpkgsrc -i" and then "make update" each package, I noticed 
> some packages got updated more than once. There were e.g. 3 packages 
> (didn't write down which ones) during which update, Firefox and 
> Thunderbird got rebuilt. Is this normal?

Yes. Use pkgdepgraph, pkgmanager, pkg_chk or whatever tool you might fancy in
order to figure out a more ideal build order. Check the archives; this 
question popped up just a few days ago.