Use Gmail’s “Basic HTML mode” by default.
Besides Gmail’s standard web interface, Google also offers a “Basic HTML mode” which until October 2021, users could select as their default view. Although the Basic HTML mode itself is still available, Google removed the option to choose it as the default view. It is still possible to use it, e.g., by clicking on a briefly displayed link in the lower right corner of the browser right after logging into Gmail, or by appending “ui=html” to the URL manually. Both options are cumbersome.
This Add-On automatically redirects browser requests to the Gmail inbox from the standard view to the Basic HTML mode.