So a few days ago I downloaded Firefox Developer Edition (v58 then) in a home folder and executed it from there directly. I created a .desktop file to have a launcher, which pointed to the executable in the appropriate home folder, and when I opened for the first time it prompted me to make it the default browser, so I did - Firefox from repos is also installed, but I planned to stop using it and start using developer edition instead.
So the problem is the following. I have set that when Firefox opens up - that is, when I first open it after I open my pc- a number of urls are opened in tabs - let's think of them as "Starting page tabs".
Suppose I have some messenger app open - not in firefox, a seperate app. Signal Messenger, Discord, Skype for Linux etc.
When someone sent me a link to open, when I clicked on it to open it what happened was that a new tab with the url opened in an already open firefox window. And that was the desired action. That was the case, while I was using Firefox stable version.
When I started using Firefox Developer Edition, and set it as default browser, that is not the case any more. Every time I try to open a URL sent in an outside messenger application, what happens is that the sites I have set as "Starting page tabs" open up inside an already open Firefox window, and the url I wanted to go to is not opened at all.
The system is Ubuntu 16.04 with Unity.