www/firefox has been changed to a release candidate version

www/firefox which was on version 52.0.2 is now 53.0 RC3
www/firefox: update to 53.0 (rc3)

More candidates are likely to come before the official announcement (aka
the rush hour) scheduled on 2017-04-19. This one intended to pick up upstream
security fixes earlier while looking for downstream regressions.

Changes: https://www.mozilla.org/firefox/53.0/releasenotes/
Security: 5e0a038a-ca30-416d-a2f5-38cbf5e7df33
MFH: 2017Q2 (piling up)

That is not acceptable from my point of view. A release candidate (RC) should be a separated port like devel/firefox or named with an obvious versioning like www/firefox53.

To make it clear: A port that is a release should not be converted in a port holding a release candidate.
Developers and maintainers (should) know the difference and why such versions should be separated.

Disclaimer: This should be taken by no means for criticizing the maintainer's work, which is outstanding for porting this huge and complicated port.
This has been this way for quite some time now. I agree with you, but you can just wait with upgrading until the release comes out next week or so.