This should take care of the width if added to userstyles.css: Code: #inbox[style*="320px"], #composeview[style*="320px"], #conversation[style*="320px"], #moreviews[style*="320px"] { width: 100% !important } I revised it to be less possible to affect other sites.
#inbox[style*="320px"], #composeview[style*="320px"], #conversation[style*="320px"], #moreviews[style*="320px"] { width: 100% !important }