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The short answer is, if you are unable to upgrade to the latest operating system (which I am assuming your computer is too old for?), then yes. Additionally, if you go the new browser route you'd need the latest or the closest to latest browser possible with consistent updates (ie: make sure that OS support hasn't been dropped by the browser provider).

The longer answer is that your OS has been unsupported by Apple since October 31, 2019. At the end of the day, ProBoards (or really, any other company/service) cannot guarantee that anything, especially if supplied by a third party will function on their platform on any OS that is no longer supported with the latest updates to its applications, even should they work on other parts of the internet. If you are no longer able to update your browser, you run a huge risk of not being able to use ProBoards or the internet as a whole as eventually sites will drop support for deprecated OSes and browsers because the cost of upkeep and security on unsupported platforms is exceedingly risky and expensive (both in time and money).

I would also encourage you to consider an update just as an overall best practice. If your OS is no longer supported by Apple, then you're going to, in the long run, miss out on critical security updates, which could potentially have incredibly negative consequences.

I don't mean this in a 'ProBoards doesn't care about your financial situation' way, but rather that planned obsolescence is an unfortunate reality of the tech world, and Apple is notorious for it. As such, it's always advised that you upgrade once your system is no longer supported, or at best run the risk of the internet slowly ceasing to function as expected, and at worst expose critical information to the whims of malicious parties.

It may also be worth noting that Mac OSX and macOS (and OSX) are in fact the same thing. Apple rebrands its operating system nomenclature every few years. Mojave, El Capitan, etc are simply the version names of the operating system that Apple uses, that may or may not also accompany a larger product rebrand. It's effectively the same as how ProBoards iterates with code names, like Blue Dragon / Hydra (v5) and Phoenix (v6). It's all still ProBoards, though, just like it's all still [Apple's operating system].