Why can't on install packages for 5-current on 5-release?

[Ben Washington-Yule]

Hi all,

I'm beginning to understand the diffs between -STABLE, -CURRENT and 
RELEASE but I have a question regarding packages. I am using 
5.2.1-RELEASE, and "pkg_add -r mozilla" installs version 1.5 or 
thereabouts of mozilla, 1.7 being the latest stable release. I have 
noticed that there is a much more recent (1.7) package of mozilla 
available from "packages-5-current"; can I install this package on my 
5.2.1-RELEASE computer?