Fortunately, updating to Apple's newest operating system, Mac OS X Mavericks 10.9, is free. MORE: Have 1 in 5 Macs Just Become Vulnerable to Attack? Check that your computer's hardware is capable of running OS X Mavericks.
If your Mac isn't new enough to run Yosemite, then unfortunately it's not capable of running an Apple operating system that's still fully supported.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 to help you get from 10.7 to 10.11.As for i OS (and you should be running the most recent version of i OS 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 i OS device will allow you to sync your bookmarks and other data between devices (Sync your Firefox bookmarks and browsing history in i OS) Read this answer in context 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 to help you get from 10.7 to 10.11.In recent history, Apple has only patched operating system vulnerabilities for the current and two previous versions of OS X.Still older Macs can't even be upgraded to Lion, meaning they'll be stuck with Snow Leopard (version 10.6.8) or some earlier version of OS X.
Those who are unsure which Mac model they own may find Every Mac and apple-history to be useful sites.