While the FastaDumper plugin works for a site viewed from the US (presumably with browser prefs set to prefer English), it does not work when the browser preferences are set to prefer another supported language. Presumably, this is because the URL parameters are being translated into the input tags "value" attribute. Not sure what version of GBrowse this is (it's GBrowse 1.X though). This bug does appear to affect GBrowse2 as well though (tested on Wormbase.org).
I will investigate. Should be an easy fix.