On the Firefox page (https://portableapps.com/apps/internet/firefox_portable/) under the "Legacy Versions" section, there are links to old versions, but they are incorrect, e.g. for Firefox 3.6.28, it links to https://portableapps.com/apps/internet/firefox_portable/localization#leg..., but only FF v24 is available for download from there.
It should point to here:
http://sourceforge.net/projects/portableapps/files/Mozilla%20Firefox%2C%...
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We're actually going to be removing that as there's no reason to use 3.6 anymore.
Sometimes, the impossible can become possible, if you're awesome!