https://portableapps.com/support/firefox_portable#plugins
It seems to me there's some outdated information that could confuse people.
The first suggestion is to "try our easy to use Flash installer for Firefox Portable," but after following the link and reading all the discussion, it sounds like it was a 0.1 release two years ago that no longer works, due to changes in how Adobe handles things.
The next suggestion is "Flash is available as an extension. Just click the link." But that link is now dead.
Further suggestion: If you do drop the link to the "Flash Plugin for Firefox Portable 0.1 (Functional Test Release)" page, I would suggest you grab timdawg's suggestion for a batch file, and incorporate that into the information on the support page.
Thanks for listening!