There's no specific indication what happened to the Firebolt after it fell from the sidecar of Hargid's motorcycle.
Ultimately there are only three options, none of which are explicitly confirmed or denied in the text;
- The firebolt was destroyed in the fall.
This seems pretty unlikely. The brooms are described as being well made and tightly bound. There's no particular reason to assume that it would break on impact even if dropped from height.
- The broom survived and was collected by the Ministry of Magic (or Harry) after the battle of Hogwarts.
This is quite plausible. The Ministry are tasked with enforcing the Statute of Secrecy which would certainly include collecting magical artefacts.
- The broom came into the possession of a muggle who has no idea that it's magical.
This is the most likely option. Given the speed at which they were travelling (and the cloud cover and the height) Harry is highly unlikely to know where the broom landed which would hamper his ability to find it.
It's clear from HP1 that while a broom may be a magical item, without a competent wizard to command it, it'll just sit on the floor doing nothing so the most likely outcome therefore is that a muggle found it and that it's sat in a cupboard somewhere.