It is always good practice to backup your user data such as bookmarks. See Back up and restore information in Firefox profiles for details of how to do this.

Depending on how Vista is re-installed, you may have to re-install Firefox and other applications. You should backup any documents and other important data you have as well before the operating system is re-installed.

I am more than willing to do this but I, not being very computer literate, don't know how to back up my Firefox bookmarks. I tried highlighting them from the drop-down on the tool-bar. That wouldn't work! I really don't know any other way! HELP again and please accept my great thanks for your help so far! Kim

The Back up and restore information in Firefox profiles shows how to backup all of your Firefox user data. If it is just bookmarks that you want see Restore bookmarks from backup or move them to another computer.

You can backup the bookmarks in the Bookmarks Manager.

You can export the bookmarks in Firefox to an HTML file:

Bookmarks > Organize Bookmarks > Import & Backup > Export HTML

You can also create a JSON backup:

Bookmarks > Organize Bookmarks > Import & Backup > Backup

An HTML backup doesn't support tags and annotations, so if you want to preserve those then you need to use a JSON backup.

Restoring a JSON backup will replace all existing bookmarks, so existing bookmarks are lost.

Importing an HTML backup will merge the imported bookmarks with existing bookmarks, so you do not lose the bookmarks that you already have, but you may get duplicate bookmarks if some already exist.

See also http://kb.mozillazine.org/Backing_up_and_restoring_bookmarks_-_Firefox