It seems that whenever I click on a link in mail from kmail, Firefox comes up right on top of kmail, covering the mail, even though I have several other desktops available. Even if I already have Firefox open in another desktop, it gets moved over to the kmail desktop from its current location. I've seen this both in Kubuntu and OpenSuSE. I would guess that the fault lies with kmail, not Firefox, though I can't be sure of that.

Is there any way to change this behavior so an open Firefox window stays where it was when I click on a kmail link (and opens the link target in Firefox, of course).

Not sure but ... try holding down the <CTRL> or <SHIFT> key when clicking. That sometimes works (though not in mails from LQ ).

I'm not in front of my machine now, but there's one place in the KDE Control Center where you can configure your default browser and mail client. Instead of putting just Firefox as the default browser, try putting something like firefox -new-tab. Clicked links should now open in a new tab in an existing firefox window.
I had something similar for konqueror, so it ought to work for you too.

I went poking around in the KDE Control Center for something having to do with email, as you suggested. The Email tab is located under Internet & Network, but when I click on it I get only the Welcome screen on the right. That would seem to indicate that I'm missing a module -- but which one? I did several searches but none of them came up with anything relevant to the problem.

Okay I am in front of my comp now so I can help. The one I wanted you to go to was in the Control Center under KDE Components->Component Chooser->Web Browser.
In there, Select the radio button that says 'Open http and https URLs... in the following browser'
and type the following in the text box right beneath it:I tested it with Kmail open in workspace one and an existing firefox window in workspace 2. Clicking on a link in Kmail opens a new tab in the existing Firefox in workspace 2.

Thanks - I found just what you were referring to, and that did the trick. A nuisance vanquished at last!

I've just upgraded to OpenSUSE 11.0, and firefox -new-tab no longer causes Firefox to open a new tab in a different desktop window. The Firefox version is 3.0. I tried firefox --help from the command line, and -new-tab isn't listed as an option.

Any ideas about how to get the new-tab behavior back?

I don't have firefox 3 on my linux box, but firefox -new-tab google.com works fine on my windows laptop at least. So I'm not sure what's wrong with yours.. how about switching to konqueror?
Perhaps you're running a beta version, maybe you could try an upgrade? Before you do that, see this link. There seems to be a command to open a new tab in an existing instance of firefox.

It works on my computer also, if I call it using Shift-F2 or from a shell. However, if I call it from a shell I get these messages:So apparently kmail is involved in moving firefox from its current window to kmail's window.

Can you try deleting /tmp/orbit* and then check again? (Or preferably just move them to some other name or location)