With the Github Bugzilla Tweaks add-on, there's now a button on Github pull-request pages that allows you to submit the request as an attachment on a bug in one-click.
To make it work, title your pull request starting with "bug ######", replacing # with the bug id from Bugzilla. The add-on logs into Bugzilla with your credentials (found in the Firefox password manager), and attaches a small HTML file that loads the pull request into the Bugzilla attachment page.
This allows reviewers to comment on your pull request from within Bugzilla itself.