Quantum is the release name for Firefox 57. It looks like people are referring to the new browser by either name, and we have tags for both. It is probably useful to have both tags, but they should be synonomized.
firefox-57 usage far exceeds firefox-quantum, so I was going to suggest making firefox-57 the primary. However, it looks like Mozilla may use Quantum to differentiate the revised code base from the pre-57 versions. We already have a question tagged firefox-quantum that refers to v58.
Since the technology change is what differentiates 57+, and subsequent version numbers won't merit new tags, firefox-quantum is probably the better primary tag.
Update: after @Nemo's comment, an even better choice for the primary tag might be firefox-57+. That should be pretty universally understandable, including by users who aren't familiar with "Quantum".