Isn’t there already a text area resizer add-on for Firefox?  Yes, there is. Text Area Resizer & Mover however adds some more functionality and class to an already great Firefox add-on idea.
To use the add-on, just resize by right-clicking anywhere inside of the text area.  With this, dragging becomes an really easy task and the often sensible site layout remains untouched.  Here is more help on how to use this extension from the author.
Additionally you can move whole text [...]