I’m sure its common knowledge by now that whatever you write in text boxes on customer support chats can be seen by whoever is on the other side, without or before hitting send. Don’t you think that’s a breach of privacy?! I imagine it isn’t too difficult to implement a fix for it: The browser (like Firefox) could choose not to upload the user input to wherever the website links to, without user input (like click a send button).
The Firefox extension API explicitly requires user actions before an extension can do things like open popup windows.
Open notepad or similar locally, write your message, think about it, delete, go back and edit, when you’re ready, copy and paste into the chat window. You can save your side of the chat so that you have a record of what you said.