If you’ve ever wanted to be able to use tab characters in multi-line textboxes in Firefox, then you are in luck because somebody created a Firefox extension that allows you to do just that.
By default it just maps the tab key to actually insert a tab character:
The nice thing about this extension is that you can assign a custom key to insert the tabs if you don’t want to lose the normal tabbing between fields functionality.
It would probably be best to map another unused key to insert tabs so that you don’t break the regular tabbing. The one thing I did notice is that you can insert tabs, but you can’t do shift+tab to remove a tab like you can in many applications.