I'll assume you're using NS 7.1, Raven. If not, go and get the upgrade from the NS website . . . for both machines.
The export/import process is different from the days of NS 4.x, when the Bookmarks file had to have a .ldif suffix to be moved and incorporated into any new profile.
I'll take a stab at how it works "today" even though my experience was with NS 4.7.8--->Mozilla 1.3--->NS 7.1 and it was simply automatic . . . I still marvel at "how did they/it do that?"
Anyhow, on your 400 you need to create a file of your existing URLs, which will carry the .html suffix.
Under the BOOKMARKS menu--->MANAGE BOOKMARKS--->open BOOKMARK MANAGER--->TOOLS--->EXPORT and you will be given the option to create the file and SAVE it. Do so.
Then in your 1.25's NS 7.1 go through the same process of opening Bookmark Manager's Tools, except this time (of course) choose IMPORT. As soon as the file created on your 400 "arrives" onto the 1.25, it will be automatically added to the existing bookmarks therein.
If you don't like the "mix" that results, you can, in your NS PROFILES subdirectory, rename your "previous" bookmarks file as BOOKMARKS.OLD and the newly installed BOOKMARKS.HTML will become your default list.
RHP
PS: For your Address Book(s) . . . in MAIL AND NEWSGROUPS--->TOOLS you'll find an IMPORT option that will lead you through a fairly simple process, if you know where your ADs are on the 400 Machine.