https://portableapps.com/u3apps/internet/browsers/firefox_for_u3
The download link is broken 
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That link takes you to the U3 download site.
FF 1.5 for U3 or otherwise is no longer supported, it is unlikely the link will be restored.
The most current version they have of FF4U3 is 2.0.0.4
Tim
Things have got to get better, they can't get worse, or can they?