The NameSync addon is unsigned, and as such you'll need to have a version of Firefox that allows for unsigned addons to be loaded.
The ability to load unsigned addons was removed in Firefox 44, which means you're going to have to download an older version of Firefox.
Below is a mirror for the Firefox 43 apk file. It has been scanned and used successfully, but use at own risk.
Make sure to uninstall any newer versions of Firefox from your phone before trying to install Firefox 43.
Firefox 43: Install
When you've got Firefox 43 (or older) up and running, tap the link below to install the addon.
Mobile NameSync: Install