A fircu can be detected easily by regular people; a fircu usually has an obsession for something, has problems comunicating and/or usually overreacts extremely when someone ask him something normal.
The word originated after heavy discussions on IRC
Jim> Can I get a screwdriver?
Retailer> What kind of screwdriver do you want, i got 40 different screwdrivers. I have the Philips 1, 2 and 3, flat, torx. Don't bother me with your ignorance about screwdrivers and say exactly what you want. I do NOT want to waste my time explaining to you.
Example 2: You on IRC:
Mike_1> m3h pwnd at GTA-SA
Mike_1> You are all t3h suxor
You> Stfu fircu
The retailer from example 1 or the irc user called 'Mike_1' in the second example are perfect illustrations of a Fircu.