Mozilla.com website does
not recognise [tt].dmg[/tt] file extension.
In the Mozilla.com, I got a similar page as beanboy89's, except it says "1.5 for Mac OS X, English (9.5MB)" and still with an ugly non-Apple display.
Then, I clicked "Download Firefox", and I received following:

They (mozilla.com) released Firefox 1.5 for Mac,
without configuring the MIME type for [tt].dmg[/tt] correctly.
Of course, I know how to download files from websites when they do not set the MIME type correctly in the server, but they should've configured this properly.
UserAgent: Mozilla/5.0 (Macintosh; U; PPC Mac OS X; en) AppleWebKit/416.11 (KHTML, like Gecko) Safari/416.12