In order for the web client to display desktop notifications, the respective permission must be granted in the browser. If this permission was denied initially, it can be granted later as follows:
Chromium/Google Chrome
Close the web client
Insert “chrome://settings/content” in the URL bar, and press Enter
Under “Notifications”, click “Manage exceptions”
Change the Behavior of “https://web.threema.ch/” from “Block” to “Allow”
Firefox
Close the web client
Insert “about:preferences#privacy” in the URL bar, and press Enter
Under “Notifications”, click “Choose…”
First, select “https://web.threema.ch/”; then, click “Remove Site“ and “Save Changes”
Open the web client, and grant permission to send notifications
Opera
Close the web client
Insert “opera://settings” in the URL bar, and press Enter
First, click on “Websites”; then, under “Notifications”, click “Manage exceptions…”
Change the Behavior of “https://web.threema.ch/” from “Block” to “Allow”