What version of Vugen you are using?
Firefox version of LR is built-in version.
VuGen 11.0 comes with Firefox 3
VUGen 11.5 comes with Firefox 8
VuGen 12.0 comes with Firefox 22
If you are using old VuGen version, your application may not work with Firefox old version, please try with VuGen 12.0
Anyway, I made a quick try with this url and it worked fine: http://www.asp.net/AjaxLibrary/AjaxControlToolkitSampleSite/ModalPopup/ModalPopup.aspx
If your application available somewhere, please share so I can try.
Thanks!