When I switched from using my ISP's mail to using Gmail, I found myself no longer using Apple's Mail.app at all. I don't want to outright delete it, as it's how I get my ISP mail, but I don't want it to launch unless I specifically need it. Trouble is, in Firefox the 'Copy Link Location' item in the contextual menu is right below the 'Send Link' item. And if I go to copy a URL from a link and miss the right option by a pixel or two, I get the bouncing postage stamp and have to wait until it's finished doing its thing. (It doesn't help that I'm in the process of switching from mouse to Wacom tablet.)
So I went into Automator, and built a workflow containing one item: Finder, Get Selected Finder Items. I left all the fields blank, then saved it as an application. Once done with that, I went into Mail.app's Preferences, and chose the workflow application I had just created as my default mail reader. That way, if I accidentally hit the 'Send Link' item, it just opens that meaningless app and closes it just as quickly.
[robg adds: Another way to disable this, specifically within Firefox only, is to enter about:config in Firefox's URL bar. This will bring up a page of defaults; in the filter box, enter mailto. Three entries should show, including network.protocol-handler.external.mailto. Double-click this entry to set it to false, then close the window. While this will disable the Send Link option, note that it will also prevent any mailto links on a page from working. I'm sure there may be other solutions as well...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060306202702398