Steps to reproduce:
- Type in some text
- Select some text, ensuring that the orange/yellow selection adjusters are visible (i.e. don't just select a single word where Firefox doesn't show these)
- Insert a URL using the link button
As you'll see, the selected text isn't made into a link, instead the URL is inserted right at the beginning of the text.
Interestingly when you select just a single word where the orange/yellow selection adjusters don't appear, the text linking works correctly. Well, almost - because if the linked text is at the end of a line, anything you type in after that is added to the linked text i.e. it becomes part of the same link.