Is there a way to increase the width of the left-side label menu in Gmail? Maybe with the help of some Greasemonkey script?
Okay completely misread the question the first time. Let's try again!
This can be done using the stylish extension for firefox. Once installed (it's in the status bar) use it to write a new blank style. In the content area put:
Obviously you can change 250px to however wide you want the labels menu. Save that and you should be good to go.