I have been looking for a way to do this since I upgraded to the Firefox 0.9.x series and in the process, I must have destroyed my Firefox profile many times over.
I have Firefox set as my default browser but opening links from external clients, particulary XNews or MS Outlook always results in a New Window. I didn't like this behaviour, I wanted Firefox to open external links in the current browser but in a new tab.
I first installed Tabbrowser Preferences but this hosed my profile (I think I may have installed the one compatible with Firefox 0.8 because I went to the old Firefox extensions site, then I tried updating TBP with the correct version from within Firefox's extension manager), I rebuilt the profile and installed TBP 0.6 and my profile got hosed again, I rebuilt again and installed TBP 0.6 again and again in different user accounts and still had to rebuild the profile everytime. I gave up on Tabbrowser Preferences and tried Tabbrowser Extensions but I noticed almost immediately that my Bloglines Subscriptions were no longer responsive so I had to remove Tabbrowser Extensions (this was mentioned in Bloglines News on 17 June 2004 - no permalinks unfortunately).
Next, I loaded up about:config and ensured that advanced.system.supportDDEExec was set to True (setting it to False will open up a new window when a link is clicked from an external client).
Today, after another round of googling, I found out that the same author who made the Tabbrowser Preferences also offers Tabbrowser Preferences Lite and this one is reputed not to destroy profiles. So after remembering to back up my profiles this time, I installed TBP Lite, closed and reopened Firefox, set the option in the extension to "Load external links in New Tab" and... problem solved!
The author mentions on his site that TBP 0.6.8 is due in August, but I think I will be staying with TBP Lite... at least until the next Firefox upgrade.
I also think that Firefox should be designed from the start to open links from external clients in a new tab; this functionality should be offered as an option and not as an extension; I have been using this browser back in the days when it was called Phoenix (I started with Phoenix 0.5 though I only recently made Firefox my default browser) so I was prepared to find a solution to my little problem but I am not sure a long time Internet Explorer user is prepared to give up on IE after his profile gets hosed by an extension.
Now I must remember to back up the profile after making this post...