User Agent: Mozilla/5.0 (X11; Fedora; Linux x86_64; rv:84.0) Gecko/20100101 Firefox/84.0

Steps to reproduce:

Create a nested list like:

<ol>
<li>11111</li>
<li>22222</li>
<li>33333</li>
<ol>
<li>11</li>
<li>22</li>
<li>33</li>
<li>44</li>
<li>55</li>
</ol>
<li>44444</li>
</ol>

Actual results:

The last item is rendered as number 6, when it should be 4. This is fixed on Firefox rapid release channel, so a backport to ESR is appreciated as this affects Thunderbird HTML emails.

Both list don't need to be ordered, the nested one could be unordered and the problem with the last one the example still persist

Expected results:

The last item should have the number 4

This was fixed in bug 1548753. It's up to relman / the TB folks to request uplift, I guess.