I ran into this problem too and, following your lead, jumped ship to slimbox.
I added 'slimbox' folder to components/com_rsgallery2/lib and extracted the css and js folder to here.
I then modified the basic template from:
$js1 = "<script src=\"".$mosConfig_live_site."/components/com_rsgallery2/lib/lightbox++/js/prototype.js\" type=\"text/javascript\"></script>";
$mainframe->addCustomHeadTag($js1);
$js2 = "<script src=\"".$mosConfig_live_site."/components/com_rsgallery2/lib/lightbox++/js/scriptaculous.js?load=effects\" type=\"text/javascript\"></script>";
$mainframe->addCustomHeadTag($js2);
$js3 = "<script src=\"".$mosConfig_live_site."/components/com_rsgallery2/lib/lightbox++/js/lightbox++.js\" type=\"text/javascript\"></script>";
$mainframe->addCustomHeadTag($js3);
$css = "<link rel=\"stylesheet\" href=\"".$mosConfig_live_site."/components/com_rsgallery2/lib/lightbox++/css/lightbox.css\" media=\"screen\" type=\"text/css\" />";
$mainframe->addCustomHeadTag($css);
To:
$js1 = "<script src=\"".$mosConfig_live_site."/components/com_rsgallery2/lib/slimbox/js/slimbox.js\" type=\"text/javascript\"></script>";
$mainframe->addCustomHeadTag($js1);
$css = "<link rel=\"stylesheet\" href=\"".$mosConfig_live_site."/components/com_rsgallery2/lib/slimbox/css/slimbox.css\" media=\"screen\" type=\"text/css\" />";
$mainframe->addCustomHeadTag($css);
Voila. Lovely lightbox++ style popups in IE and Firefox.