User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:83.0) Gecko/20100101 Firefox/83.0

Steps to reproduce:

1. Create new message (Ctrl-N)
2. Click UI button to Apply Numbered List
3. Type the following keystrokes to create a trivial numbered list with a numbered sub-list:
   X [Enter]
   [Tab] A [Enter]
   B [Enter]
   [Enter]
   Y

Actual results:

List came out as:

1. X
   1. A
   2. B
2. Y

Expected results:

1. X
   1. A
   2. B
2. Y

In true comedic form, Bugzilla inserted its own ol and li tags for my manually numbered list in the original description of the bug. To restate - the last item in the list came out as a 3, not a 2 - that is the bug.

Please refer to the attached screenshot to see it in action in Thunderbird.

Also note: It appears the bug is with Thunderbird's counting of the li tags in a nested ol tag in general.

The markup it generates is correct as viewed with another mail reader (gmail), but the numbering in both the Thunderbird message compose window and message view pane is incorrect as described, so this may be a bug with the underlying html parser or renderer is my amateur guess.

I think, bug 1548753.