1. Open a compose window.
2. Click in the message body.
3. Click the enumerated list button.
4. Type "foo" and hit enter.
5. Type "bar" and hit enter.
6. Type tab, type "baz" and hit enter to create a sub-list with one item in it.
7. Type shift-tab to indent back out to the main list.

At this point the number of the list item you're about to type into should be 3, but it's (incorrectly) 2.

Regression:

https://hg.mozilla.org/comm-central/pushloghtml?fromchange=6af3543d4d15b4b0f98fbb3323c9e8c8271faec1&tochange=bca07912f1dd450719ea332823565918e87e4b6f

I am not sure this is actually a comm-central regression. As far as I can tell this is the mozilla-central push log:

https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=5dc0652cd024fef30217ac3e6087b7a32f16967f&tochange=3d5cd10cb1b20c1f83a189fbeb2e22f470bac0ec

To be clear, I first noticed this issue on trunk, but it's broken in the released TB68, since the regression happened on March 25.

Thanks for finding the regression range. Perhaps bug 288704? Which has several regressions filed against it. For example bug 1545746

There's a huge discussion about that, go and vote for bug 525888.