You've probably done what I did, set Gnome default browser to Firefox, set KDE default browser to Firefox and still, when you click on a link and Epiphany or Google Chrome comes up showing your page...
To check what "system default" is type:
$ xdg-mime query default text/html
google-chrome.desktop
then to change it, create firefox37.desktop in /usr/local/share/applications:
[Desktop Entry]
Comment[en_US]=
Comment=
Exec=/usr/firefox37/firefox %U
GenericName[en_US]=
GenericName=
Icon=/usr/firefox37/icons/mozicon128.png
MimeType=text/html
Name[en_US]=Firefox 3.7
Name=Firefox 3.7
Path=
StartupNotify=true
Terminal=false
TerminalOptions=
Type=Application
X-DBUS-ServiceName=
X-DBUS-StartupType=
X-KDE-SubstituteUID=false
X-KDE-Username=
Mimetype
should be set to what you intend to use it for...and set it:
$ xdg-mime default firefox37.desktop text/html
$ xdg-mime query default text/html
firefox37.desktop
happy browsing...