You're comparing apples to oranges. WebOS was closed source with limited hardware vendors. FireFox OS is open source, open to all hardware vendors.
Good platforms tend to do well in the marketplace because they attract investment from the outside world. Android is beating iOS because it's a better platform for vendors. FireFox OS is not only a better platform for vendors than iOS, it may turn out to be a better platform for devs than both iOS and Android.
One big difference from WebOS is that the company behind FirefoxOS will not likely abandon it within 12 months of kicking it off. 4th or 5th place in the market might represent enough scale to be considered a "success" as far as Mozilla is concerned.
The phones are also being built by multiple large ODM's, and not just one company that's barely staying afloat (Palm) or a company suffering at the time from a horrible CEO (HP)...
It is great that Mozilla is fighting for openness, but in terms of what street joe and street jane want, I think it will join WebOS in a few months.
Given my experience in telecommunications I don't see how sales are going to push these devices out of the shop when users come asking for other OSs.