The fireflies inhabit aroundlive near a brook at the foot of a small mountain.

I usually pass through the mountain to go to university, but I didn't know about the fireflies.

I think it sounds better to put the "I" before didn't. Otherwise, it sounds kind of strange.

Here's an example where I don't put "I" in front of a verb:

"We drank and chatted all night."

I don't need to put "we" before "chatted" because it is close enough to the first "we". However, in your sentence, I think "didn't" was too far away from the other verbs/pronouns in the sentence.

I barely remember I watcheding fireflies as a kid, so I was moved by how beautiful it was at thate time.

The slow, flashing glow was faint, but fluorescent, which was flashing slowly.

Writing "...,which was flashing slowly" sounded odd here, so I moved those adjectives to the beginning, so that they could directly describe the glow.

Also, if you write anything like "It was x, but y..." like "the glow was faint, but florescent", then you need to add in this comma before "but".

I hope we can see fireflies at theis spot forever.