Flash in Firefox

It's not entirely clear if you are seeing a notice to upgrade Flash, or that your version of Flash is not compatible with the version the site is built to use.

In either case, the current version of Flash for Linux is the version provided during normal system updates of Ubuntu. You may need to update.

However, Adobe stopped adding features to the version of Flash it releases for Linux a few years ago. Since then, it's only added security patches. It gets the same security fixes, when appropriate, as the other versions.

It's easily possible that a site built to expect the current version of Windows Flash will not work with Linux Flash. If a site is telling you that current version is vulnerable, I'd guess it's looking only for Windows version numbers and is clueless about Flash on Linux.

Google made a separate deal, as you probably know, to package a more uptodate version of Flash with the version of Chrome it distributes for Linux.