Actually, I think it is connected to MicroG somehow. Using a LineageOS with GAPPS doesn’t cause the reboots. Also, I fiddled around a little and figured out that the crashes of Fennec only occur with an active internet connection (mobile data or wifi). But please be careful trying that yourself. After starting Fennec in flight mode, I deactivated flight mode a minute or so later. The phone rebooted instantly. After the phone startet it seemed like Fennec is automatically started in background as well. Then the phone rebooted again. I had some kind of boot loop. I solved it by entering my SIM pin and activating flight mode very fastly and then uninstalling Fennec.
All recent updates of Fennec didn’t solve the problem for me until now.
Btw: I’m not using IceCat anymore as it seems to have been removed from F-Droid Repo and doesn’t receive any updates anymore. @DietmarP I tried DuckDuckGo as well, but for me that browser has some quite annoying features. Im going with SmartCookieWeb browser from F-Droid for now, it has similar features like Fennec.