"Doom!" is a very seductive narrative that we fall to regarding virtually every change in our environment. Sometimes reasonably, as with AGW, but other times just taking natural variations and screaming end times...then just going quiet when they recover again as we move to the next cause.
Observational and experimental studies provide evidence hat ALAN adversely affects firefly populations. Several studies have shown negative correlations between high levels of ALAN and firefly abundance.
The citation for the statement "Firefly populations are declining worldwide, for a variety of reasons" is a link to firefly.org. That is a small, Texas group that appreciates Fireflies, which is nice. But it is home to no studies, no research, no analysis. Nor does it (that group, or the wiki page that was cited to apparently refute my personal anecdote...) cite the publication you mention. So I'm not really sure what we're discussing here.
I'm not disagreeing that there is a decline for a multitude of reasons, or that light is probably bad, but if someone says "my yard has no fireflies the apocalypse is upon us" it probably isn't credible.
Spot on, this is usually the real explanation for the typical 'I saw none'/'oh but I saw a bunch'/'I saw more than ever' rethoric applied to one single year or even multiple years of observations. Thinking of it, goes for basically all slowish trends related to nature. Unless you're approaching it scientifically, i.e. actual systematic counting which is hard btw, it's oh so easy to be fooled. That being said, see other post: yes fireflies are in decline (just like many other insects unfortunately) and light pollution is a factor.