www.crutchfield.com
IMO probably the best car stereo place available. I dont know exactly what size speakers are in your Firebird, but I'm going to guess that the front ones are 4x6 (or maybe 5.25 rounds) and the rears are 6x9's (or maybe 6.5 rounds). Thats a standard combo, but check and make sure.
You're probably not going to get better than a 2 way for under $60 a pair. Not saying you couldn't but I'm just saying it's probably not likely. Basically, a 2 way is the bass with a mid range speaker. A 3 way generally has that plus a tweeter for higher notes.
This is a reasonably decent 6.5 inch round speaker for $50. Now, this is just a personal thing, but I like speakers with a wider range of sound reproduction than most people, also like better sensitivity (measuered in dB, higher =better). That said, there is a whole range of speakers similar to this in the 50-70 price range. Step up a level and you'll get things like rubber/urethane surrounds (last longer) better woofers (material wise) and better tweeters and mids. Search around and see what you can find.
Here is a pretty dang good set of speakers for $60. It's got the nicer surrounds, same sensitivity as the earlier link, but it doesnt have as wide of a sound range and the tweeter isn't quite as good. In my experience, it's all a balancing act of what you want. You may find that you dont care about sound range, and just want pure power handling capability. It's all personal taste. My recommendation would be to go to a local electronics shop (even circuit city or best buy) and listen to different speakers and see which ones you like. Make not of that speakers specs, and see if you can find something similar (or maybe that one even) online. Compare it to others and make your decision.
ANyways, sorry if it was a little longwinded, but this is only scratching the surface. THere is so much more you can do with car audio. Hope this helps, and best of luck.