Due to your O/S being x64 the latest version will if Auto update be x64 FF there is no way around this unless you offline install a x86 FF(but future updates will revert back to x64). I used the x64 FF so far and my Addons have worked without problem. Most likely eventually x86 will cease to exist and your only choice will be x64 because most O/S are going x64 for stability and security. When x64 installs any x86 will be removed or cease to work in this instance ththis isn't a problem that's what happens. This is something your going to have to decide.