To elaborate, this is an issue in networks that have configured a route for the origin different from the default route. What happens is that Firefox picks the IP address of the default route's interface and uses it as its source IP address. But then the OS sends it via the route that has been configured for the destination IP address. The destination network will then discard the packet since the source IP address does not lie within its network. If that's a bit confusing, basically it boils down to this:
Default Route's Interface (1) has address (A).
Other Interface (2) has address (B).
Origin has been fetched via (2).
Firefox picks (A) but it's being sent via (2).
The result is that if STUN and TURN servers are reachable via (2), no srflx or relay candidates can be gathered, limiting the application to host candidates.