AT&T prevents the loading of non-Android market applications on their phones. There are ways to get around this, use your favorite search engine to look up 'AT&T sideload'. The phone you have is an ARMv6 phone. Mozilla produces an experimetal build that can be downloaded from [https://wiki.mozilla.org/Mobile/Platforms/Android#ARMv6_.28experimental.29]. This build may crash or have performance problems on your phone.