It is common for commit messages in Firefox to contains strings like Part 1, Part 2, etc. See this push for bug 784841 for an extreme multi-part example.

When code review is conducted in Bugzilla, these identifiers are necessary because Bugzilla orders attachments/patches in the order they were updated or their patch title (I'm not actually sure!). If part numbers were omitted, it could be very confusing trying to figure out which order patches should be applied in.

However, when code review is conducted in MozReview, there is no need for explicit part numbers to convey ordering because the ordering of commits is implicitly defined by the repository history that you pushed to MozReview!

I argue that if you are using MozReview, you should stop writing Part N in your commit messages, as it provides little to no benefit.

I, for one, welcome this new world order: I've previously wasted a lot of time rewriting commit messages to reflect new part ordering after doing history rewriting. With MozReview, that overhead is gone and I barely pay a penalty for rewriting history, something that often produces a more reviewable series of commits and makes reviewing and landing a complex patch series significantly easier.