I am interested in how you got FF to open NS Mail on Send Link.
FF as a standalone browser, will only open the system default mail client. Your wife may not like having NS mail as the default. That would mean any mailto links she clicks on in her browser would open NS. Check this out carefully to make sure its working for her.
The other problem is that NS Mail/News is always going to open any links in the NS browser. Its a suite, so there is no way around that.
There are a few options available to you if it does not. One might be to take a look at the SeaMonkey application suite:
http://www.seamonkey-project.org/
It is a modern version of Netscape, using the same rendering engine as Firefox. Since it a Netscape model suite, links in the mail component will open the SM browser. Send Link will open the SM Main client. You can do this while leaving the system defaults as they are, in your wife's preferences. That would be the easiest option.
Another is to set up separate user accounts in Windows for you and your wife. Then you can each set the default applications as you like. This will require logging off and on, but its probably the best option.
The Launchy extension *might* help. It hasn't been updated in a while, but it does give you some options as to which browser or mail client is called on a link.
I would tend toward SeaMonkey, but then I have always loved the idea of the combined internet application suite.
Lee
UserAgent: Mozilla/5.0 (Macintosh; U; PPC Mac OS X 10.5; en-US; rv:1.9.1b3pre) Gecko/20090223 Mnenhy/0.7.6.0 SeaMonkey/2.0a3