The only links Thunderbird opens internally are to certain pages run by the project to serve things like information and add-ons. Additionally RSS feeds render HTML pages if the default settings are changed.

Given the above, what use case is there where is it actually all that relevant?

Matt said

The only links Thunderbird opens internally are to certain pages run by the project to serve things like information and add-ons. Additionally RSS feeds render HTML pages if the default settings are changed. Given the above, what use case is there where is it actually all that relevant?

Via some add-ons (e.g. G-Hub Pro) its possible to open pages such as calendar etc in a tab. Some of the internally opened pages also open in tabs (as you mention - addons) which can cascade to other sites. Sites such as calendar and others e.g. Hangouts Chat utilise web push notification API but using e.g. https://www.bennish.net/web-notifications.html the 'authorise' box never appears.