Download Mac OS X Lion 10.7.5 in ISO, DMG and Torrent! Since you are unable to get Mac OS X Lion 10.7 from Apple, I have decided to upload here. Here are 2 links to get OSX Lion 10.7.5. Mac OS X Lion 10.7.5 is an update to OS X version 10.7 which is generally called Lion. The Mac OS X Lion 10.7.5 consolidates general working system fixes which will redesign the strength, security, and likeness with your Mac OS.
This is a Firefox support forum, not a mac one, so we won't be able to help with the specifics of updating your mac, but Apple has a good support page set-up at http://www.apple.com/osx/how-to-upgrade/ to help you get from 10.7 to 10.11.
As for iOS (and you should be running the most recent version of iOS for security reasons) both Firefox and safari are equally secure, as they both use the same backend (Apple won't allow a competing backend on ios), but if you are using Firefox on your computer, using Firefox on your iOS device will allow you to sync your bookmarks and other data between devices (Sync your Firefox bookmarks, open tabs and browsing history in iOS)