Research for disabling that feature on websites with enabled auto focus led to a Greasemonkey user script that would do exactly what we have been looking for earlier today: A script that would automatically jump to the first text field on selected websites without user interaction. A Greasemonkey script for that purpose is great because of the following reasons:

• It is not limited to the Firefox web browser but can also be added to other web browser supporting Greasemonkey or those scripts.
• It can be enabled on specific domains (or all).

A user wanting to install the script needs to download Greasemonkey or a compatible program. Once Greasemonkey or its equivalent are installed the Focus First Textbox script can be added to the web browser. The last step involves adding domain names or strings where this user script should be active on.

A right-click on the Greasemonkey icon in the status bar of Firefox and the selection of Manage User Scripts will open the configuration. Select the Focus First Textbox script from the left sidebar. A click on the Add button next to included pages will open a basic form that can be filled with a domain name.

The script becomes active on newly added pages after reloading them.