I totally agree with wd5gnr
Here are the minimums for Firefox:
Minimum Hardware
Pentium 233 MHz (Recommended: Pentium 500MHz or greater)
64 MB RAM (Recommended: 128 MB RAM or greater)
52 MB hard drive space
Linux
Software Requirements
Please note that Linux distributors may provide packages for your distribution which have different requirements.
Linux kernel - 2.2.14 or higher with the following libraries or packages:
glibc 2.3.2 or higher
XFree86-3.3.6 or higher
gtk+2.0 or higher
fontconfig (also known as xft)
libstdc++5
Firefox has been tested on Linux Fedora Core 4
Minimum Hardware
Intel Pentium II or AMD K6-III+ 233 MHz CPU (Recommended: 500MHz or greater)
64 MB RAM (Recommended: 128 MB RAM or greater)
52 MB hard drive space
Mac Operating Systems
Mac OS X 10.2.x and later
Minimum Hardware
Macintosh computer with a PowerPC G3, G4, or G5 processor
128 MB RAM (Recommended: 256 MB RAM or greater)
4 GB hard drive space
Default Firefox is aprox 5 Mb and with only the verry necesarlly extensions on it I am sure that it comes in fornt of Opera.
Still here are Opera's minimum:
Minimum configuration for Opera 7 without Java:
Any system running Windows 95 (or higher) operating system
9 MB of free disk space (maybe less if installing from CD)
9600 bps modem
Recommended configuration for Opera 7 without Java:
Pentium class system with 16 MB of RAM and at least 12 MB of free disk space
Windows 95 or higher operating system
28800 bps modem or higher connection
Please note that Java requires at least a Pentium 166 MHz processor, 32 MB of RAM and 65 MB free hard drive space to run.