I'd like to wrap up the thread that started with my post:

> Okay, I want thunderbird & firefox to "cooperate" under kde, so I did
> the following, as I read in earlier posts:
> In thunderbird:
> network.protocol-handler.app.http --> mozilla-firefox
> (same for https)
> In firefox
> network.protocol-handler.app.mailto --> mozilla-thunderbird
>
> And... it works, up to a point. If I click a web link in an email
> message, firefox starts up and displays the link. If I open another
> link, however, with firefox sitll open, rather than opening the new link
> in a new tab (as I've set in firefox preferences), or even in a new
> window, or replacing the current window, a new copy of firefox is
> spawned (? usage) and I'm asked to choose a profile. Is there a way to
> avoid this?

With help from HJohnson and CTower, I have a solution. HJohnson
suggested that my installation might be broken, and CTower suggested
that the script I was using was incorrect. It turned out that both were
right... I re-installed both firefox and thunderbird into /opt using the
distributions right from mozilla.org rather than the slackware packages
from linuxpackages. And I tried:

In thunderbird:
network.protocol-handler.app.http --> /opt/firefox/firefox
(same for https)
In firefox
network.protocol-handler.app.mailto --> /opt/thunderbird/thunderbird

And it "just works." No additional scripts needed at all. If I open a
web link from thunderbird, and firefox is already open, it opens a new
tab. Clicking a mailto: link in firefox opens a new compose window in
thunderbird whether or not its running.

Nick is happy; thanks for all the help!

N