Firefox 3 requires Mac OS X 10.4 or above.
Another option is TenFourMac, which is based on Mozilla 2.0 and Firefox 4 code. Mac OS X 10.4 users, and 10.5 users with a PPC Mac, can download the latest Firefox 3.6.xx release version for Mac OS, currently available from here or from the Mozilla CDN site (Firefox 3.6 requires Mac OS X 10.4 or above).