I have been trying to load the add on "compactheader 1.2.4" but when I try to add it through Mozilla Thunderbird, open add ons from the too menu it does not show up even though I have downloaded it to my download file. When I try to open it directly from the download folder using Firefox 3.6.10 it says it is not compatible with Firefox 3.6.10. How can I get the headers to reduce to one line so they don't take up so much space on Thunderbird.