That is highly dependent on OS and plugins/add-ons.
I am running FF 4.0 on my antique 2001 laptop with only 192 MB memory and often have 50 MB left over! That is a crippled Pentium II also.
I keep getting plug-in errors with my most current version. Anyone have any suggestions on this—was thinking of going to Chrome, but if I can figure out the issues, will go with Firefox 4...perhaps the issues will be solved with the new version?