The Firefox shell for iOS devices required '''iOS 8.2''' at minimum when it was first released in Nov 2015 and even then you needed iOS 9+ to fully use some features such a Private browsing mode then. Firefox 1.1 was released [https://blog.mozilla.org/blog/2015/11/11/firefox-users-can-now-choose-their-favorite-browser-on-ios/ globally Nov 11, 2015] and not in limited market as 1.0 was. from https://www.mozilla.org/firefox/ios/1.1/system-requirements/ iOS 8.2+ iOS 9 The current Firefox 12.0 for iOS requires iOS 10.3 at minimum. The link for 12.0 does not exist so here is one for Firefox 11.0 for iOS. https://www.mozilla.org/firefox/ios/11.0/system-requirements/ The Firefox for iOS is more of a shell as it does not use the rendering and JavaScript engines that Firefox for Android and desktop uses thanks to Apple's restrictions on what can be used. I have not seen any plans to try and support old discontinued versions like iOS 5.1.1 in which support ended back in 2014.