if you've previously installed firefox, you can still find it in your apps page otherwsie its no longer available.

for those with problems installing other updates could you reboot and try. there are times if you've done an uninstall of an app the cache is not free yet hence the installation from store fails. also, while installing could you try keeping the installation dialog as your active app instead of sending it to the background.

at times when installing apps the system will close all other opened apps to complete installation so if you're using an app it can't close it hence the installation failing.