Looking on the internet deeply has found these results:

Firefly is the most popular phrase on the web.

Lightning bug 1457,000 results on the web	Firefly 142,800,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
She's got more spark then a barrel full of lightning bugs. Actually, lightning bugs are in the beetle family. You know, like, if there's cool stars out, or... lightning bugs. They're cross fertilizing pears with apples and goats with sheeps, tobacco plants with lightning bugs The difference between the right word and almost the right word is the difference between lightning and the lightning bug. Y'all look out for the lightning bugs after sunset. Mark Twain said that, The difference between the almost right word and the right word was the difference between the lightning bug and lightning. No, those are lightning bugs.	Roommates agree that Friday nights shall be reserved for watching Joss Whedon's brilliant new series Firefly. His Excellency, Rufus T. Firefly. He begged the Fox network not to cancel Firefly. He owns Fox, and they cancelled Firefly. My dad read by firefly as a kid, I thought we could take a ride to Firefly Hill. ...like a firefly lights up at night. Firefly did a movie to wrap things up.

Lightning bug

1457,000 results on the web

Firefly

142,800,000 results on the web

More popular!

She's got more spark then a barrel full of lightning bugs.
Actually, lightning bugs are in the beetle family.
You know, like, if there's cool stars out, or... lightning bugs.
They're cross fertilizing pears with apples and goats with sheeps, tobacco plants with lightning bugs
The difference between the right word and almost the right word is the difference between lightning and the lightning bug.
Y'all look out for the lightning bugs after sunset.
Mark Twain said that, The difference between the almost right word and the right word was the difference between the lightning bug and lightning.
No, those are lightning bugs.

Roommates agree that Friday nights shall be reserved for watching Joss Whedon's brilliant new series Firefly.
His Excellency, Rufus T. Firefly.
He begged the Fox network not to cancel Firefly.
He owns Fox, and they cancelled Firefly.
My dad read by firefly as a kid,
I thought we could take a ride to Firefly Hill.
...like a firefly lights up at night.
Firefly did a movie to wrap things up.