+1
Can you check this out?
The following sentence is from a grammer test.
It asks us to select between 'firm' and 'firmly.'

* We could not walk, the fibers of our clothing would fall apart and we would find it difficult to build houses because all of the nails would pop out as fast as we hammered them in. Screws and bolts would not hold [firm / firmly] because there would be no friction to hold them together.

The answer is 'firm.'
And I understand the answer because in this context 'hold' means 'remain.'
But I'm wondering why we can't use 'firmly.'
If 'remain' is intranstive verb, why can't we use 'firmly'?

There's an advertisement which seems support my point.

Inserting PlierNon-serrated/SerratedThe most clever plier for inserting auxiliaries. Looks like a cutter but doesn't cut. This finely tapered instrument is ideal for placing all types of attachments. Serrated tips hold firmly while applying pressure. Doesn't damage the wire.
+1
"to hold firm" in this context means that the screws and bolts are solidly in place and are stable. It is a recognised collocation for carpentry.

The advertisement you quote is not the most grammatical I have seen - I don't think it is written by someone skilled in English.
Comments  
Thanks, Feebs 11.

I appreciate it.