open_external Background:
Other applications will often use the system’s default browser to load web pages. When a Mozilla-based browser is running, it can catch links sent by other programs. This preference determines how Mozilla handles those links.
Possible values and their effects:
1 = Open links from external programs in the current window.
2 = Open links from external programs in a new window.
3 = Open links from external programs in new tabs in an existing window. (Default in Firefox)
While this an 'under-the-hood' option that can only be accessed via about:config, it is an important one because in my browsing routine, I want URLs that I click in my email program or instant messaging program to bring up links into new windows, not buried as one of the many tabs in one of the many open browser windows. Also, when it is directed to a tab instead of a new window, the page does not pop-up and into focus for me to read and I have to go searching for it. So i prefer open_external=2.
After months since the update to Firefox 3.5, I have finally found a solution. An AddOn created by mrchucho.net called Tabs Open Relative (Modified) 1.0 (a modified version of the Tabs Open Relative AddOn), which restores the open_external behavior!