In Cinnamon, I could not choose Firefox as the default browser from "System Settings" -> "System Info" -> "Default Applications", which caused links opened from other applications to use Chromium or another installed browser instead of Firefox.

After Googling a bit, I found out that this is due to the .desktop file from the Firefox package missing 2 entries for it to be considered as a candidate for the default Web application.

So here it is:

• edit /usr/share/applications/firefox.desktop (as root)
• add x-scheme-handler/http;x-scheme-handler/https to the MimeType entry
• run: sudo update-desktop-database

and you should now be able to select Firefox as the default Web application.

Hope this helps someone.