An earlier extension exists for this, so I probably will not develop this one further: https://addons.mozilla.org/firefox/addon/window-titler/

Click the extension's toolbar button to modify the current title preface (because this extension does not request tabs permission, this will not show the current preface, if any).

Currently, your text is always preceded by [ and followed by ] and then separated from the page title by a space. In a future update, you'll be able to modify that to suit your style.

Install

• License: Mozilla Public License 2.0
• Weekly downloads: 0
• Average daily users: 7
• Rating: 0/5 of 0 ratings
• Created: 2021-03-07 19:15:56
• Last updated: 2021-03-08 07:10:16
• Homepage: https://github.com/jscher2000/window-title-preface
• Support site and email
• Orig: https://addons.mozilla.org/en-US/firefox/addon/titlepreface-add-name-window/
• API: window-title-preface@jeffersonscher.com