Running an outdated browser leaves you exposed to all kinds of security vulnerabilities. For example, in the last version alone, the Firefox developers fixed 21 security holes (11 of which were labelled as "high" risk and 2 of which were "critical" risk). So, if you were to skip just 5 updates, you'd be exposed to more than a hundred documented and well-known issues, which often makes you an easy target for malicious parties.

Aside from that, each update comes with a variety of performance enhancements and other bug fixes.

My assumption is that you didn't post here from the computer that you say is "too old to be updated", since your system details says you are running Firefox 66.03. Naturally, if you are running another computer that's old and can't take the update, you won't be able to advance past whatever version you have on that system.

This will leave you exposed to tons of security issues, but you don't really have much choice unless you were to upgrade your computer.

However, if you update Firefox on your primary computer (which I highly recommend), it won't block you from using the older version on your other system. Mozilla doesn't block access to old versions of Firefox. Although I'd never recommend it, you could theoretically download Firefox version 1 if you wanted to.

Hope this answers your question.

Sorry, duplicate comment.