I think that I would like to revise my original comment. I said that a G3258 was optimal. I guess, I think that's optimal, as far as entry-level bang-for-buck, if you can get it with a mobo combo, and then add a SATA6G SSD (as low as $35 recently for a refurb 60GB Corsair, for example).
But as far as usability, I could go lower. I would say, a G1610/1620 Ivy Bridge dual-core 2.6/2.7Ghz, which is probably similar to a Core2Duo 3.0-ish. An SSD, if you can fit it into the budget, makes things so much better too.
You can get along with 4GB of RAM, but after leaving the PC running for a week or two, memory usage in Waterfox creeps up enough that I would prefer 8GB.