It could be practical if you could somehow tell from the history viewer if an entry is currently open in any tab. Consider for example that you have three browser windows open with many tabs in each. You're thinking of closing one tab, but out of mistake you close the whole window instead. To retrieve the lost tabs, best method is to open the History viewer and sort by access date. Now here's a mixture of addresses you already have open in the other two windows, but also there are the lost pages. Since you probably can't tell right off which are which, you will open all relevant entries, resulting in duplicate tabs.

Suggestion:
There could be either some icon (like the "new" indicator in a mailbox) telling that this entry is currently open in a tab.
Or the History window could have a filter option:
Show...
inactive addresses
active+inactive addresses.