Depends on a number of factors, like which version exactly of Red Hat are you using. One way is to use wget, for example; wget URL_PATH_and_full_filename_of_the_rpm_package
FYI: thunderbird came out after RHL9 support ended by Red Hat over two years ago.
So that leaves downloading it from;
http://www.mozilla.com/thunderbird/ Hint: click on the download link.