IIRC, topic branches were used because:
- Mercurial named branches were half baked at the time and I was advised against them by folks who knew hg better than me.
- We were already using separate clones for security update versions (3.5.x and 3.6.x) so we just kept doing the same. Some tooling could be reused.
- Rapid release was a big change and we didn't want to disrupt mozilla-central dev. Having rapid release in different clones got us out of the way and reduced the risk of negatively affecting development. m-c was the domain of developers, the rr repos were the domain of the release team.
