Great but ... Rated 3 out of 5 stars

Thanks for creating an add-on for a feature that should have been built into all Mozilla products. There are variations of programs available for MS Windows that attempt to do this. Many of us don't use Windows, and the sources are questionable.

My suggestion would be to replace -no-remote with -new-instance. This would fix the issue of not opening links with the running instance of Firefox. If the user chooses to keep separate instances of Firefox running rather than closing the currently running one, links would still be opened by the first instance by default (the operating system shouldn't matter).

You can set this as the default action. In a later version, you can add an option to change the default behavior, or choose the behavior in the dialog box, or both.

I'll probably hack the code myself, but this issue keeps me from giving a higher rating.

This review is for a previous version of the add-on (1.5.0.4.1-signed). 

This a great suggest!
I will implent it in the next update, maybe the best solution is to use -new-instance as default behaviour for Firefox/Thunderbird 13 or higher, giving the user the possibility to switch to the old "-no-remote" if s/he prefers.
Thanky you for pointing this!