For an impression how much users want to select and copy from inbound message headers, see the ancient parent bug:
Bug 61497 - [SM] Can't select text in message headers / copy subject
**42 duplicates** were filed against that bug, most of them demanding that *all* of the headers should be selectable for copying, i.e. including *recipients*. However, bug 61497 only fixed half of the problem (selecting subject), the other half was moved to this bug 167010 (originally `SeaMonkey`, then `MailNews Core`, now `Thunderbird`).
For an impression how much users want to select and copy from inbound message headers, see the ancient parent bug:
Bug 61497 - [SM] Can't select text in message headers / copy subject
**42 duplicates** were filed against that bug, most of them demanding that *all* of the headers should be selectable for copying, i.e. including *recipients*. However, bug 61497 only fixed half of the problem (selecting subject), the other half was moved to this bug 167010 (originally `SeaMonkey`, then `MailNews Core`, now `Thunderbird`).