This works in UX Nightly too! Rated 5 out of 5 stars
I just tried this in UX Nightly (version 24.0a1), and it works! I tried changing "browser.tabs.tabMaxWidth" all the way up to 500 prior to installing Custom Tab Width, but only Custom Tab Width makes it work! Nice.
I didn't need this until the UX Nightly build. In Firefox 21, my setting in about:config was still working. Well done, Dao Gottwald!