Hi, it's right there is the title "Mark Folder Read"", not "Mark selected messages read". If you want to mark a few messages read, you need to select them and with right click Mark > Read.

Hope that helps!

Sorry. Maybe I didn't explain it properly.

I want to mark all the messages in the search folder as read but not the entire folder that the search folder is a subset of.

Let's say I've created a search folder that has the main inbox as the source. The search parameters are set to filter for all emails from microsoft.com. The search folder then fills up with all the messages from Microsoft.com. If I then try to mark that search folder with "Mark Folder Read" then it not only marks the all the messages in the Search folder as read, but everything in the inbox it's based on too.

This is very specific to Search Folders, not normal ones.

Ah I got it now. I did the same and I see what you mean. Can you add a bug here: https://bugzilla.mozilla.org/ I agree this is bad from consumer perspective. I think (maybe I am wrong) that the search folders are "virtual folders" and every action towards them goes to the main folder as instructions.