No it is not. It is the past tense and past participle of the verb to slither. It can be a verb form, a participial, and very rarely an adjective (more frequently used is the present participle, slithering).
The verb form slithered does not have an adverb form. There are adjectives slithery and slithering.
He slithered helplessly down the slope.
No, it is not an adverb. The word dollar is a noun. There is no adverb form.
The word he is a pronoun; an adverb modifies a verb or an adverb.
The word not is an adverb. The word there can be an adverb. The combination "not there" is a compound adverb.The homophone phrase "they're not" includes a pronoun, a verb, and an adverb, because the adverb not has to modify an understood adjective or adverb (e.g. "They're not colorful).
The verb form slithered does not have an adverb form. There are adjectives slithery and slithering.
I slithered down the muddy bank of Indus river.
glide
There are 11 syllables in "beside your shoe it slithered round."
No, "slithered" is a regular verb. Regular verbs form their past tense by adding "-ed" to the base form. In this case, the base form is "slither," and the past tense is formed by adding "-ed" to get "slithered."
Slid
Slithered
The slit grew wider as the material stretched. He cut a slit into the side of the animal.
In the sentence the slimy, green snake slithered through the tall grass the nouns are snake and grass
He slithered helplessly down the slope.
Slunk, crawled, or possibly whickered.
Snakey sally slithered stunningly