Macro add-on . Does it exist?

Add-ons Development
#1

Is there a add-on that works like a macro?
For example, I type in message composing process a word PREFACE and the add-on would replace it with a text( for example) like this
Hello,
How are you?

#2

I don’t know whether there are any of these for Firefox 57-65 yet.

There are a couple for Google Chrome that have mixed reviews, and could possibly be ported over to Firefox ( Auto Text Expander is open source , ProKeys is not ).

I use Clippings, but insert from the context menu. I haven’t set up hotkeys.

#3

Take a look at Auto Hot Key (AHK). It is not limited to the browser, but will do what you want and more.