er how do you thread the mail and know this is happening if your mail is not sorted into a threaded view?

I have the View>Sort by set to Date, Descending, Unthreaded. I can see the list of all of my most recent emails (usuall 10 to 12) in that folder, including both read and unread. If I hit "R" when a single unread email is highlighted (and displayed in the lower pane) any other unread messages of that thread (i.e., those with Re: [Same Subject]) turn from bold to regular font in the list at the same time, as if I had selected them as well, which I hadn't.

are you using any addons? Conversations perhaps?

No add-ons (I double-checked-- the only add-on listed is "Default 24.3.0 - The default theme") From your questions, it sounds like you haven't been able to recreate my symptoms. More information that you haven't asked about that may be applicable: User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.3.0 The email threads/exchanges that I am having the problem with contain multiple replies to the same original. I could understand if marking the most recent message as "read" marked the earlier ones also, since the newest one contains all of the previous messages. However, even marking the original (oldest) one marks all of the subsequent messages read, so that isn't the answer. Also, the effect is only one-way-- marking as "Unread" any previously marked "Read" message changes ONLY that message to Unread. This provides me a workaround BUT ONLY IF I am paying attention to which messages get erroneously changed in the first place.

"If I hit "R" when a single unread email is highlighted"

According to this document:

https://support.mozilla.org/en-US/kb/keyboard-shortcuts

R marks a thread as read. Where did you get the idea that R marks a single message as read?

Try M to toggle the read/unread status of an individual message.

"Where did you get the idea that R marks a single message as read? "

From the "Other actions" dropdown box: "Mark as Read" - The "R" is underlined. Underlined character has always meant shortcut key. This menu is located on the window of a single email, so if, as is Mozilla policy, menus are context sensitive, typing R should mark only that item, correct?

I'm not sure that accelerators as seen in menus are directly interchangeable with keyboard shortcuts. For instance, F9 in the Write window will toggle the display of the Contacts Sidebar. It won't do that in any other window, so it is a context-sensitive shortcut. Most menu accelerators require you to go through the sequence that leads to the menu being displayed; therefore one accelerator letter can have a different meaning in each menu. Some RAD/GUI development tools automatically select accelerators for the coder, and in general are not aware of any letters having been already assigned as global shortcuts.

I glanced at my menus to see if it suggested any shortcuts for marking as read, and here in the linux build, there is no indication of accelerators or shortcuts in that specific menu. Having found M being given in that document, I tried it but discovered that it clashed with one of my add-ons that uses shift-M to move a message to another folder (along with shift-G to go to another folder.)

If you open that menu, then yes, R should mark as Read, so long as nothing else has bound that keystroke. But if it's an accelerator, rather than a shortcut as listed in the page I gave a link for, it's valid only when that menu is actually visible.

Did you try shift-m?

Shift-M does nothing (at least in Thunderbird) but M by itself does mark the message and only that message as read. I'm glad you brought that to my attention.

I do think that it would be beneficial to change the entry in the "Other actions" dropdown list to say "Mark thread as Read" instead of "Mark as Read". This wouldn't change any hotkeys or accelerators, but would more correctly describe the action resulting from clicking that choice (or from typing R.)

Just for completeness, a copy of the windows Menu for Mark. Notably no mention of M at all to mark as read :( But is does toggle the read status read/unread

I also noticed that selecting more than one message and M toggles the read/unread status of all of them

Any idea what the mark as read by date is?