I'm not sure, bmarkus. While modifying an ISO can indeed be seen redundant if booting from USB, maybe thats exactly what harassedstudent wants to know how to do.
I used to have a USB stick with several ISOs which could be booted via GRUB2.
And the topic title says "NOOB proof way to add firefox to tinycoreplus iso
[...] how to add firefox to my tinycoreplus iso. So that it is present when I run tinycore off my USB stick. [...]
harassedstudent, can you be more specific? How exactly are you booting TinyCore?
A frugal USB installation of TinyCore with persistancy (a /tce directory) is probably the most noob proof way to carry Firefox around. No iso involved though.