I keep getting message that firefox will no longer be supported on my macbook.

You appear to be using Firefox 152 on macOS to post this thread.

Firefox ever since 116.0 and later still requires macOS 10.15 (ten.fifteen) or later to run and I have not seen any plans to require a more current macOS to use Firefox. https://www.firefox.com/firefox/152.0.5/system-requirements/ and the Firefox 153.0 currently on Beta channel still requires macOS 10.15 at minimum. https://www.firefox.com/firefox/153.0beta/system-requirements/

If you are asking about the same browser/system you used to post this question, you can continue using that configuration happily going forwards.

If there's a website telling you it won't support you browser, please share the address and ideally attach a screenshot with the error message so someone can inspect what the confusion is. Thanks!