Not every new update reaches all devices at the same time
Browser vendors generally distribute new versions gradually, or step by step. Otherwise, this would overload the servers. A new release may therefore already have been published even though it is not yet offered on your device. BrowserDetails.org consequently distinguishes between “up to date”, “update is rolling out” and a genuinely outdated version.
For Firefox, both regular releases and longer-maintained ESR versions are also taken into account. Safari, on the other hand, is tied more closely to macOS, iOS and iPadOS and is therefore assessed more cautiously.
What an up-to-date browser can improve
Updates close known security gaps and fix errors. They also ensure that modern websites, forms, videos and web applications work as intended. Regular updates are therefore extremely important.
But: an up-to-date browser alone does not automatically make your device completely secure. Keeping software updated is nevertheless the foundation for safe internet use. You should also regularly install operating system updates and use up-to-date antivirus software (updates matter here too!).