[Google-Desktop] GoogleDesktop in Firefox and Seamonkey

A long preamble to a question:

I have Seamonkey 1.0b set as my default browser on a computer without
Firefox installed.Even with Seamonkey as my default browser,
GoogleDesktop would open in I.E.

So, I went to Google tech support and asked what I needed to do to get
it to open in Seamonkey. They said that they "supported" Firefox, but
not Seamonkey.

On another computer, I had both Seamonkey 1.0b and FF 1.5., with
Seamonkey as the default browser. So, I made FF the default and clicked
on GoogleDesktop. Sure enough, up in came in FF. Then I made Seamonkey
the default again, clicked on GoogleDesktop, and up it came in
Seamonkey.

As a final experiment, I uninstalled FF, and then clicked on
GoogleDesktop with Seamonkey still as the default browser.
GoogleDesktop just sat there and would not open. When I re-installed
Firefox, and kept Seamonkey as the default, GoogleDesktop opened in
Seamonkey.

Now, finally, the question:
What is the relationship between Seamonkey and Firefox that is being
expressed here?

My buddy said it was plugins, that all Gecko based browsers will use
the same library of plugins. But, I went to about:plugins in both
browsers and found nothing about GoogleDesktop.

So, I would like some technically proficient person to answer this
curiuous question.

Thanks a lot in advance.