User-Agent JS Fixer addon?
User-Agent JS Fixer does not change the oscpu only modifies some ua work well in certain pages, the only thing that works well so ua is Random Agent Spoofer but this is a very old list ua , so firefox does not work well with it, I found that now reviewing TotalSpoof called this change the oscpu but is not configurable, thanks for replying :) (sorry my English)
UserAgent with custom oscpu
someone could create a modifiable ua with modifier oscpu for firefox
(sorry for my inglish)