Some apps shut down after running few seconds

@Lidwien As pointed out here (in German) Lightning uses the old browsing engine on FP1 and is thus insecure - just like the preinstalled Browser app.*

You could try clearing the cache of all apps. To do so go to Settings > Storage > Cache and click “ok” when prompted. This will clear the caches of all apps, but keep all important personal data.

Further, I can recommend the app Autostarts, available from the F-Droid Archive. This app will enable you to disable apps from starting in the background and using up processing time and memory (and might even increase your battery time a bit).

* For more info see ✏ Constructive tips for improved security for the FP1(U)