Firefox Issue

Hi all,

I have designed my very first homepage with the twenty seventeen theme 4.7.4. Everything is running pretty good and it´s quite easy to configure. I really like WordPress. Thanks for the amazing tool.

Unfortunately I have a really big issue with my homepage.
https://srilanka-passion.de
It´s my very first homepage and it´s a summary of my Sri Lanka vacation. I launched the website last weekend and I often heard from friends that they are not able to open it.

I could rebuilt the issue, so as soon as somebody uses an old version of Firefox (tested 45.3.0 and 38.x.x) they cannot open the homepage.
They do not get any error or something like that – you can press enter when you typed in the address and nothing will happen. They can search for the website on google and the website appears – if they follow the link, they see at least a white page.. but nothing else.

Has anybody heard about similar issues? How can I resolve this issue?
I also could note that when somebody uses an old version of internet explorer that they will see my homepage on mobile design. I have added some style.css notes concerning full width and not showing sidebar on small mobile resolution.

Looking forward to hearing something from you.
I really appreciate your support in advance. Thanks and regards from Germany,
Tobi