For reference, a full fleet of five types of Mark I fleet ships and and three types of Mark II fleet ships runs at roughly 400 firepower, a single Botnet Golem runs somewhere close to 100,000 firepower, and a single spire cruiser champion with plasma modules might run at around 700 firepower, on High ship caps off of one homeworld.
I would say that firepower is not particularly useful as a means of comparison of fleet strength, though, since the firepower rating is so variable for various ship types, and the AI doesn't always release all of the ships on a given planet when you send your fleet in, and not all the threat ships necessarily attack the same world at once.
In order to see the firepower display in the galaxy map, the left-hand filter needs to be one of the filters with 'ships' in the name, and the right-hand filter needs to have one of the various options that have '[firepower]' in the name. The 'Mobile Military [Firepower]' filter is possibly the most useful of these, since it causes the galaxy map to display the firepower of your mobile military ships (blue number above a planet) and the AI's mobile military ships (red number below a planet).