User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.98 Safari/537.36

Steps to reproduce:

1. Create contenteditable content
2. Add a list, either numbered or not
3. Use indent command

Actual results:

List is wrapped with blockquote

Expected results:

List should move one level deeper (Chrome, IE and Safari do that)

Can you provide me some screenshot/s or a Test case in order to figure out what is the problem. I've tested on win.7 - on different versions of Firefox - using Sublime text as a text editor. All 3 steps described have been fulfilled; I've render the specific document on Firefox, but no issue happens.

What do you mean Sublime text as editor? Why would that matter? The issue is about execCommand commands on contenteditable element. You can check it in any WYSIWYG that uses native commands, for example this:
https://codepen.io/chrisdavidmills/full/gzYjag/

Or in Mozilla's old Midas editor:
https://www-archive.mozilla.org/editor/midasdemo/

that was the reason of what I ask for more info, because the description was not clearly. I've tried to reproduce of how I understand, without any extra info.

I was able to reproduce this issue on windows 7 on all Firefox versions: nightly 67.0a1, beta 66 si release 65 and windows 10 on versions beta 66 & release 65.
For reproducing this issue I will add a description of how should encounter it:

1. grab the link: https://codepen.io/chrisdavidmills/full/gzYjag/
2. put the cursor in front of any paragraph and hit "indent" option from the green list
3. see the results -> text appears with blockquote

&

1. select from the green list the options "insertunorderedlist" & "insertorderedlist"
2. make a list with a couple of items
3. press "indent" option
4. see the results -> text will appear with blockquote

PS: the issue can be inspected in browser console.
See the attachments

as well can be reproducible in the 2nd link: https://www-archive.mozilla.org/editor/midasdemo/