You can use code in the userContent.css file to hide/disable the resize control or use a bookmarklet

You can use code in the userContent.css file to hide/disable the resize control or use a bookmarklet to toggle the resize property.

textarea { resize:none !important; }

javascript:(function(){T=document.activeElement;if(/textarea/i.test(T.nodeName)){T.style.setProperty('resize',(T.style.getPropertyValue('resize')!='none')?'none':'both','important')}})()