Mozilla.org has released Mozilla 1.5.1, a Mac-only update to its browser suite. This version fixes an issue with downloading. From the brief note on the Mozilla releases page:
Mozilla 1.5.1 is a Mac OS X-only release to correct a downloading crash experienced by users of Mac OS X 10.1.
A Mozilla 1.5.1 download link can be found at http://www.mozilla.org/releases/