I just installed the new FireFox and by doing so, I lost a little trick that
I have been enjoying greatly so far:  whenever I click on a link from outside FireFox, I
want it to be loaded in a separate tab in an existing window and not in a new
browser.

The instructions to enable this feature are in the FAQ but a bit scattered,
so here they are in a more concise fashion:

  • Add the following to your user.js:
    // Stop reusing active windows:
user_pref("advanced.system.supportDDEExec", false);
  • This file doesn’t exist by default, so you must create it.  On
    Windows XP, it can be found at:

    c:/Documents and Settings/[user name]/ApplicationData/
Mozilla/Profiles/default/[random chars].slt/user.js
  • Download and install

    TabBrowser extensions

     
  • Select Tools > Extensions, select Tabbrowser Extensions and click the
    small Options icon.
     
  • Expand the General node.
     
  • In the Window Mode panel, select Use multiple browsers only
    for my request.

Note that the "small Option icon" in the Extensions window is not only
minuscule but doesn’t look like a button at all (no push affordance).  It’s
the label with a check mark on it.  I find this inconsistency in the FireFox GUI annoying:  why should options for extensions be in a different
menu than the regular preferences?

Anyway, FireFox is a cool browser, and even if the progress in terms of
features is pretty slow, it is still light-years ahead of Internet Explorer,
which has been stagnating for years now.

Update, 2004/08/23: I updated these instructions for the latest Firebox build since the path of user.js and the Settings option panel of the TabbedBrowsing extensions have slightly changed