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 areas to another place. Double click inside of an area and all areas will get unlocked and movable. The repositioned area will stay fixed on the place of your choice even if you are scrolling the page. Double click once more to lock the areas again. The extension’s options allow some manual adjustments (e.g. use context menu item instead of double click or additional key modifiers for mouse gesture users).

You can pickup Text Area Resizer & Mover at the Firefox Add-ons site or the extension’s official home page.