On your question, you can always check threading for a folder by clicking view>sort by and seeing if threading is ticked. You can permanently stop threading, but it's a bit of one-time work. Here is my info on threading:

This is the official (to my knowledge) documentation. Well worth the read https://support.mozilla.org/en-US/kb/message-threading-thunderbird

Simple Approach if just a few folders Here is my own documentation on threading, duplicating the above, but it may help: You need to highlight each folder and click View>sort by>unthreaded. If this applies to just a few folders, just repeat the process as necessary. If you have many folders or plan to add many new folders, you may wish a different approach, below:

FOR NEW FOLDERS See Settings>Appearance

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FOR EXISTING FOLDERS These steps may help: To set many *existing* folders, follow these steps: This must be done on an account by account basis.

1. select table view - the icon just to right of 'quick filter' button above message list pane
2. select a message folder and apply all desired attributes - View>sort by from menu bar (if not shown, click alt key)
3. click the icon at right side of column display in message list pane
4. select the entry near bottom 'Apply current view to...'
5. highlight 'folder and children'
6. move mouse left to see account name and move left again to display all account folders. Now CLICK. a small menu will appear to confirm setting all folders within that account.
7. repeat steps for each account

FOR FUTURE THUNDERBIRD INSTALLATIONS

1. -To change default for *new* folders, In config editor, you can change default that applies to NEW folders you set up:

mailnews.default_view_flags 0 =unthreaded (NOTE: Config editor is at settings>general near bottom of pane)

1. - create a simple text file with name of user.js with these lines:

    // Set default threading to UNthreaded  ###   <== this line is optional, just for documentation
    user_pref("mailnews.default_view_flags", 0);

1. - install Thunderbird and exit prior to setting up accounts
2. - copy the user.js file into the profile
3. - now, when you start Thunderbird, unthreading will be the default. Problem solved. #

Thank you for your help. I didn't realize that threading was either enabled or disabled for each folder. As it turns out, it was disabled for the Inbox, but it was enabled for Trash. I will make sure that it is disabled for all folders.

I still need to know if, when I delete an email with an attachment, the attachment is deleted too.

You are quite welcome. Yes, the attachment is also deleted.