Maui has Firefox 51.0.1 64 Bit although the current Firefox is already 52.0.1 64 Bit, the same issue has Thunderbird. I have added the Mozilla Security ppa, but I don`t get an update. The same problem as in Linux Mint. Does anyone know the reason for this? Thanks already for an answer.
We ship Firefox and Thunderbird with Plasma integration patches. Those take a little bit to build. We provide them in the plasmazilla ppa. This has a releases ppa with the tested released versions. We also provide a builds ppa there where you can see the currently building or uploaded packages. Those are the ones we test before they land in the releases archive. You can see that 52.0.1 is already there. But we are also building and testing a 52.0.1esr version.
Or if you REALLY need the newest version provided by Ubuntu (in this case 52), you can force it in Synaptic. Find the firefox package and hit CTRL+E, then select the version you want. But bear in mind that vanilla Ubuntu firefox does not contain desktop integration with Plasma.