You definitely need a new fire extinguisher, those first alert/safety firsts are one shot deals, can't recharge them. If you buy another of the same cheapo extinguishers, open the box and check the gauge before buying, there's about a 25-40% chance it's dead new in the box. The ones that say "commercial" on them are metal instead of plastic and are more reliable.
Sorry about your battery and the rant. I work with fire extinguishers and can't stand those cheap, unreliable, plastic pieces of junk. I'm glad it worked when you needed it, but I'd consider myself lucky it did.