Add code to userChrome.css below the @namespace line.
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */
See http://kb.mozillazine.org/Editing_configuration#How_to_edit_configuration_files

.bookmark-item[livemark] .bookmark-item {max-width: none !important}

I have learned how to do the trick with the user chrome but a lot of people complain about it, especially in the french version where 90% of the titles of the "what's new" default RSS feed on the personal bar, are longer than the maximum allowed, making the RSS feed unusable.