The default web browser in Debian's GNOME environment is Epiphany.

The default web browser in Debian's KDE environment is Konqueror.

These can be changed to fit users' needs.

People preferring other browsers (e.g. Iceweasel (Firefox)) have several ways to change the default browser.

GNOME

Default for GNOME sessions (system-wide)

The default GNOME browser can be changed for the whole system by reconfiguring the alternative.

# update-alternatives --config gnome-www-browser

Default for GNOME sessions (user-specific)

Use the settings under Desktop -> Preferences -> Prefered applications, the web browser tab.

Default for KDE sessions (user-specific)

Under System Settings -> Default Applications -> Web Browser, change Open http and https URLs to "in the following browser:" and enter the path of the browser's executable (for example, /usr/bin/iceweasel).

To change how htm or html files are handled when they are clicked change the preference in:-

Control Centre -> KDE Components -> File Associations -> Known types, Text -> html

Default for foreign programs (system-wide)

Programs which are not designed for the user's desktop environment do not obey the browser settings of the desktop environment (GNOME or KDE). For example, Icedove ignores desktop environment-specific browser settings. The default browser for generic applications can be changed for the whole system by reconfiguring the x-www-browser alternative.

# update-alternatives --config x-www-browser

Removing alternative browsers from the system

This method involves removing all the other browsers installed on the system, so that the desired one remains the only one. Sometimes this is not feasable (multiuser system, dependencies) and is the least elegant.

Any of the previous methods is preferred to this one, but this method can be useful for lightweight systems.