I could see Ubuntu become the main alternative to Android for manufacturers, because Canonical will make it easy to piggyback on Android drivers, so it should be very easy for manufacturers to port Ubuntu to their devices. It also gives them at least as good customization power as Android, and the hacking/ROM community are going to love it, which I think is a good way to get people excited about a device/OS online these days.
WP8 doesn't give them any of that, it's been stagnating at 2% for more than 2 years and a half, and besides perhaps helping Nokia a bit (while being a much smaller company than they used to be, so easier to be "satisfied" with it), there's nobody really benefitting from supporting it in terms of sales. So Ubuntu would be a great WP8 replacement there, and as a strong customizable alternative to Android.
As for Firefox OS, I guess it depends on how well this ChromeOS/FF OS thing will work. And if it does, it probably only works on the low-end, at least for the next 5-10 years, when data will be so cheap, that using web apps won't be a problem anymore (although you can still use "native" web apps in FF OS, just like in Ubuntu Touch, so I guess there's that, too).
But since FF OS is going to be used mostly on low-end devices, that means it could be on a lot of devices as well, so it could do well in terms of market share (or at least well enough to give a company like Mozilla good revenues). For the same reason, I don't really see FF OS and Ubuntu competing with each other, since Ubuntu will be more on devices with at least a quad core A9/A53 or dual core A15/A57.
The only way they are competing with each other is manufacturer's attention. Some of them may not be ready to take on more than 2 operating systems at once, and if they have already committed to FF OS, then it might take stronger persuasion to start supporting Ubuntu as well, to have an alternative for high-end Android devices.
Android will continue to compete with both, at the low-end and at the high-end, although I think Google needs to make Android 5.0 a little leaner to put it back into the Gingerbread-range of resources needed, if they want Android to go hard into the sub-$50 phone market.
WP8 doesn't give them any of that, it's been stagnating at 2% for more than 2 years and a half, and besides perhaps helping Nokia a bit (while being a much smaller company than they used to be, so easier to be "satisfied" with it), there's nobody really benefitting from supporting it in terms of sales. So Ubuntu would be a great WP8 replacement there, and as a strong customizable alternative to Android.
As for Firefox OS, I guess it depends on how well this ChromeOS/FF OS thing will work. And if it does, it probably only works on the low-end, at least for the next 5-10 years, when data will be so cheap, that using web apps won't be a problem anymore (although you can still use "native" web apps in FF OS, just like in Ubuntu Touch, so I guess there's that, too).
But since FF OS is going to be used mostly on low-end devices, that means it could be on a lot of devices as well, so it could do well in terms of market share (or at least well enough to give a company like Mozilla good revenues). For the same reason, I don't really see FF OS and Ubuntu competing with each other, since Ubuntu will be more on devices with at least a quad core A9/A53 or dual core A15/A57.
The only way they are competing with each other is manufacturer's attention. Some of them may not be ready to take on more than 2 operating systems at once, and if they have already committed to FF OS, then it might take stronger persuasion to start supporting Ubuntu as well, to have an alternative for high-end Android devices.
Android will continue to compete with both, at the low-end and at the high-end, although I think Google needs to make Android 5.0 a little leaner to put it back into the Gingerbread-range of resources needed, if they want Android to go hard into the sub-$50 phone market.