Weighing in at a 9.5MB download, you will need at least 25MB of spare hard disc space, and lots of RAM to load it (Peter's help file suggests around 30MB to load, which can get to over 64MB whilst using it). You'll also need at least RISC OS 4 with long filenames and more than 77 files per directory. It is apparently possible to persuade it to load in RISC OS 3.7, but it's unstable if you do so.
People downloading this beta release of the browser are urged to fully read the !Help file within !Firefox before attempting to use it, as it contains usage information and bug reporting information.
This particular release is based on the Deer Park Alpha 1 browser, which was recently released by the Mozilla developers for testing purposes. Although it's not currently as responsive as other native RISC OS browsers, it does show that a lot of work has gone into porting the browser, and that there is still some work to do.