Build ID: 2003102905 (Mozilla 1.6a) Confirmed on Linux (Suse 8.2) and Win2k. My guess is OS=All. Hit CTRL-H to bring up history. Select an entry in the history list. Select Edit. Delete host and Delete domain are greyed out. I also reported this on bug 148959 but discovered I couldn't edit severity.
*** This bug has been marked as a duplicate of 148959 ***