The Mozilla blog has an announcement
that Firefox will be moving to 4-week release cycle, starting in 2020.
“Shorter release cycles provide greater flexibility to support
product planning and priority changes due to business or market
requirements. With four-week cycles, we can be more agile and ship features
faster, while applying the same rigor and due diligence needed for a
high-quality and stable release. Also, we put new features and
implementation of new Web APIs into the hands of developers more
quickly.” The Firefox ESR (Extended Support Release) release cadence
will remain the same.

Source: LWN.net – Moving Firefox to a faster 4-week release cycle