If you rely on a niche corporate intranet tool or a deeply customized add-on built before 2017, the only way to keep using it is with an older Waterfox version.
Waterfox, originally founded in 2011 by Alex Kontos as a high-performance 64-bit alternative to Firefox, chose to split its development. While the "Current" branch (now G-series) followed modern Gecko standards, Waterfox Classic
Waterfox 56.2.14 (2019) or Waterfox 2019.10 (the final pre-Classic branding release).