Theplaceforitall.com has made available for download a two-in-one Mac OS X and Windows PC cross platform version of Firefox version 1.5 Portable edition so you can take your bookmarks, extensions, history, cookies, cache, web search engine favorites, and saved passwords and run this single piece of software on any Mac or Windows PC from a USB drive.

Clicking PortableFirefox.exe on a Windows computer will launch Firefox using the files in your portable profile. On a Mac, clicking “START OS X Firefox” will launch a script that makes your Windows portable Firefox profile folder compatible with Mac, and then it lauches Firefox using that profile.

There’s also a Mac and Windows PC Cross platform version of Mozilla Thunderbird at http://www.theplaceforitall.com/portablethunderbird/