Should not be an issue, particularly, with current Waterfox.Waterfox uses Firefox code ... sooner or later their code, when updated to follow The Internet, and changed by Mozilla will change something and that your extension(s) may no longer work?
Certainly could be an issue if Waterfox (any browser that currently supports Legacy extensions, like SeaMonkey, for that matter) follows the Quantum "path" - unless he is (they are) able to somehow maintain the pieces that Mozilla has pulled out (which might be just about, everything).