It's a Firefox Add-ons called Typefill

It is for those who are tired of typing the same replies, sign-offs and notes over and over, that is what Typefill is for. You save text with a short code, type the code in almost any text box in Firefox, and the full text goes in. If parts change each time, like a name or a date, it asks you first.

Free with no limits, snippets sync through your Firefox account, and nothing you type leaves your browser. The free version is open source (GPL-3.0). There's a Pro for if blocks, formulas and Google Sheets data, but you don't need it.

Feedback very welcome.

The code is here if you want to check: https://github.com/lagudafuadtosin/typefill

Add-on: https://addons.mozilla.org/firefox/addon/typefill-free-text-expander/