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Pro Can switch operating systems easily
When buying a phone you typically buy into an OS, then only upgrade that OS to a new versions; never switch to a different OS. GeeksPhone was built to help open up the ecosystem, allowing for entry of other OSes without committing users to the investment & risk of getting a new phone with an OS with which they're not yet familiar.
To make this possible there are options to "install another OS" or "install zip from external SD" easily accessible from the phone.
Pro Comes with a pre-rooted Android
GeeksPhone comes with a pre-rooted Android operating system that doesn't even includ the default Google apps.
Pro First device to activelly support FireFox OS
FireFox OS, also known as Boot to Gecko or B2G, is a new operating system from Mozilla, that aims at creating an affordable smartphone solution for the emerging market.
To help Mozilla achieve this goal, GeeksPhone offers FireFox OS build as self-contained downloads for flashing the phone.
Con Can't dual boot
Whichever OS you choose to install, it will overwrite the previous one.