Please explain what you mean by "group" and "this group function". I haven't noticed any change myself in the ability to multiselect and operate on the selected messages.

Hi Zenos: what I mean is that when I highlight a set of emails - several from the same writer, or on the same subject -- and then try and drag them as a group to deposit them in a file (in the list of files down the left-hand side of the screen), the group of files disintegrates, and only the one my cursor is resting on is moved over into the file. Previously all the files would move 'en bloc', even if they were scattered here and there in my Inbox, and they did so for years. Thunderbird seems to make changes from time to time in some functions without warning, e.g. "un-reading" emails in a file all of a sudden. This is one such change.

I had no change in this function since acquiring Thunderbird several years ago till now; that's why it's so perplexing!