Fedora 25 Spin: Cinnamon
After some research I ended up here (this forum)
I removed Firefox completely and I'm using Iron Browser.
After moving the iron-browser.desktop file (created with menu libre) to /usr/share/applications, it was
not showing in Preferred Applications
It seems that using DNF for installation,you will find the application in the application list of Preferred Applications.
For Iron Browser, I had only a tar file for Fedora, After un-tar, I moved the directory to /opt.
In the directory /usr/share/applications you have a file called "mimeapps.list". Using Nano I replaced firefox.desktop with iron-browser.desktop and restart my laptop
And it is showing in the application list and selected for Web : Iron Browser