applications-defaults: Set text/html to firefox

Instead of opening a new browser window when a user with default
settings used `ipython notebook`, a Geany Window with a temporary .html
file was opened, because the setting for text/html was not defined.

Set text/html application to firefox.

applications-defaults/defaults.list

@@ -3,4 +3,4 @@ application/pdf=evince.desktop
 application/x-bzpdf=evince.desktop
 application/x-gzpdf=evince.desktop
 application/x-xzpdf=evince.desktop
-
+text/html=firefox.desktop