User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:75.0) Gecko/20100101 Firefox/75.0

Steps to reproduce:

I defined an HTML enumerated list with multiple ol types:

<ol type="I">
  <li> one </li>
  <li> two </li>
  <ol type="A">
<li> alpha </li>
<li> beta </li>
<li> gamma </li>
<ol type="i">
  <li> one </li>
  <li> two </li>
</ol>
<li> delta shows C should be D</li>
  </ol>
  <li> three shows IV should be III </li>
  <li> four shows V should be IV </li>
</ol>

Actual results:

The resulting enumeration sequence is incorrect:

I. one
II. two
A. alpha
B. beta
C. gamma
i. one
ii. two
C. delta shows C should be D
IV. three shows IV should be III
V. four shows V should be IV

Expected results:

Enumerated list should look like
I. one
II. two
A. alpha
B. beta
C. gamma
i. one
ii. two
D. delta shows C should be D
III. three shows IV should be III
IV. four shows V should be IV

Additional information:
I have attached a .HTML file demonstrating this problem.
This problem is not seen in Safari or Chromium-based browsers.

I was able to reproduce this on my macOS machine.

I used mozregression to narrow down the time window of when this bug was introduced:

 9:15.58 INFO: Last good revision: 4a692c812a3fe2f893d2a6e25b9490b38415c907 (2019-03-25)
 9:15.58 INFO: First bad revision: 0969fd6383cea58141d70fbb2d0895ccb476af6c (2019-03-26)

Here is the push log for that time period:
https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=4a692c812a3fe2f893d2a6e25b9490b38415c907&tochange=0969fd6383cea58141d70fbb2d0895ccb476af6c

Looking through the log, the most likely candidates to me seems like they might be these pushes:
https://bugzilla.mozilla.org/show_bug.cgi?id=288704

6c2e7cfa54847968b0e98fc4e3719c2a633276ca	Mats Palmgren — Bug 288704 part 3 - Make nsBulletFrame use the built-in 'list-item' CSS counter and remove the old implementation. r=emilio
ae4e4daebdc4531be3f55fdced39155b97645cee	Mats Palmgren — Bug 288704 part 2 - [css-lists] Implement display:list-item counters using a built-in 'list-item' CSS counter. r=emilio
84c99621920420234be5ba591afe1ed32ccbee07	Mats Palmgren — Bug 288704 part 1 - [css-lists] Add an inherited internal UA sheet property (-moz-list-reversed:true|false) to propagate <ol reversed> to its relevant decendants. r=emilio

Mats and/or Emilio, can you take a look and see if you think this patch may have caused the regression?

The html in comment 0 is invalid, in fact, the <ol type="A"> should be nested inside the <li>.

This behavior changed when we moved our list counters to use regular CSS counters, see the duplicate bug for more details.