Lexis is certified for use in Internet Explorer, and some versions of Firefox, but not 9.0 (yet?). If you ever find that the checkboxes don’t load for combined source searching, try clearing out the cache and all data except for cookies. (Hit Ctrl+Shift+Delete to bring up the “Clear All History” interface, and select everything except for “Cookies”.) Wait for the program to finish processing, log out of Lexis, close Firefox, re-open Firefox, and log back into Lexis. It worked for me, and hopefully it will work for you too. Thanks to Scott from Lexis technical support.