IIRC .com was just not available back when. Also, for historic reasons, a lot of FF sites / communities have taken the .net moniker (.network, social sites, something like that? idk).
As for the difference, there's not much of one practically. There's a number of people that would use to put ".com" behind every web address (similar to everyone putting www. in front of web address), but not sure how much of a habit that still is nowadays, given that most people don't even bother with entering internet addresses anymore and just go with google - often faster, too.
It shouldn't affect search rankings at all, the domain name is for the top level ones (.net, .com, etc) not relevant to that. I think. The penalty for changing from .net to .com would be far, far greater than any benefit of having .com as a primary address, too.
Finally, .com is kinda intended for companies / businesses.