Let me clarify.

x86 refers to 286/386/486/586/686 which is the series of Intel processor types.

Layman's terms:
It's a CPU series for Intel/AMD, but derived from Intel

ARM refers to older mobile CPU's that runs CPU instructions differently from Intel CPU instructions.

Layman's terms:
A mobile CPU that's different from Intel and works differently

In other words, x86 code won't run well, if at all, on ARM-based system, case in point, our tablets. Unfortunately, Ubuntu runs on x86 systems (Desktop/Laptop/Atom(Netbooks)) meaning the Flash Player plugin for Firefox would not work.

Mer, running off ARM, would have to have an ARM-based Firefox and an ARM-based Flash Player to get what you're looking for. It's not impossible, in fact, it's already on your devices (so to speak) in the form of MicroB, the tablets default browser.

Fennec, being developed by Mozilla, is basically a mobile version of Firefox with a different graphical interface, so it won't look like the Firefox you know in Windows/Ubuntu/Mac. It will look different and feel a bit different, but it will be the same. They will probably develop a Flash Player for the program and if not, someone else will. No worries.

Jesse~