SUMMARY Mozilla's Bugzilla and Phabricator instances integrate well with each other. In particular, I can log into Mozilla Phabricator with my BMO account and when I submit diffs to Phabricator, they automatically link to the bug on BMO (and the inverse: BMO bug links to my Phabricator diff). I really like the workflow that Phabricator and Bugzilla promote, but it can be intimidating for new contributors who may be used to a GitHub/GitLab-style workflow. Deeper integration between the tools used would significantly reduce the overhead (and possibly even enable reuse of some Mozilla VCS documentation).
FWIW KDE's migrating to gitlab and is phasing out phabricator, so this bug should probably be resolved, I guess as unmaintained? (Some bugzi installations have a resolved as obsolete option for that purpose, but I don't see that choice here.)
We now use gitlab for reviews.