It's built from Firefox and is probably still using that as the identifying useragent. The reasoning would be that Firefox is a recognizable agent and Waterfox, if used, might be misidentified as a bot or other nefarious thing.
EDIT: And, in fact, this appears to be the case. I couldn't quickly find it in the Waterfox docs but a number of web sites state that as fact as does this one:
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