digiKam uses konqueror and kmail

The kipi-plugins use the default KDE application settings to invoke web browser and email clients.

In Metadata/GPS sidebar, when I click on "More Info...", it opens
konqueror, how could I change it to another browser (firefox or anything
else) ?

Start kcontrol (or systemsettings), then go to "KDE Components/Component Chooser", navigate to "Default Applications", then click on "Web Browser" and choose "in the following browser" where you can put any command.

In Menu->Image->Email Images, kmail is invoked, but I use Thunderbird (or any other email client)

Start kcontrol (or system settings), then go to "KDE Components/Component Chooser", navigate to "Default Applications", then click on "Email Client" and select you preference

The email client can also be specified in the ->Email Images dialog itself on the mail page. The setting will be remembered by digiKam.

Note: If you use Ubuntu, the kcontrol or systemsettings menus will not be available to you, you have do do in the digiKam dialog box as described above.