Foxconn obviously think Firefox OS is a safe bet, since the company are hiring some 3,000 software engineers to help them out. Why would Foxconn need the software engineers being a manufacturing facility? Well, software engineers can write Firefox OS drivers and optimizations for devices that they build, which helps them in a big way.
Considering Foxconn are mostly known for manufacturing Apple devices (as well as numerous other devices) it is definitely an interesting move, especially to back Mozilla's mobile OS. Foxconn wants these new software engineers to be experienced in HTML5 and cloud computing, which is the future.
HTML5 experience comes in handy from a manufacturing point of view, as most devices can benefit from it - not just Firefox OS.