I just saw something that struck my interest: When I was browsing SEN my FF's status bar showed that it is "waiting for 1.1.1.4". What is behind this IP and what does it have to do with me browsing SEN?

Not sure why this would be showing up for you: https://ipdb.at/ip/1.1.1.4

Oh its the IP address of my top secret spy server. I mean, umm.

It's possible that someone linked to an image hosted there. Check the source of the page where that message is coming up for you.

Answer.

Sadly I just found out that you're pretty close to the truth. Except it's not your server, but my provider's.
I'm using a mobile connection and I just learned that mobile providers in Germany intercept the response from the webserver I'm trying to access and change the html file. Sometimes they even inject java code. This means I'm actually not connecting to the actual website but to a rigged copy! It's said to be used to reduce bandwidth by, for example, reducing a png image to a jpg. But who said I want that? And how do I know what else they are changing? Am I maybe being censored? I don't know because nobody told me about it happening.

Looking into special right of termination of my contract right now.

What in seven hells?? Sounds like some 1984s Orwellian shit going down in Germany right now.

T-dang German pinko commie bastards, ehhh!

Did everyone tl;dr the link I posted a long time ago?

The first part of the answer by CowOfDeath is quite wrong; strange that someone who knows about IANA reserved addresses would go on to write what he did. Maybe he just didn't understand what was being asked.

I did. But I only read the first response because it was the "most helpful" (granted only by 1 person). And I didn't understand much of what it does when he said it's a cache.
florx' answer was much more helpful and concise, but I haven't read it at the time.

Also there's a few other things other than caching/compressing images, which are not addressed in your link, Aristo (which are the source of my indignation). They are also injecting and executing java code on my machine and remove line breaks in the html file which can screw with site formatting, and the injected code can cause problems on java based and ajax dependent sites. Which coincidentially could be the reason I sometimes cannot connect to SEN when using the mobile connection. (But that's just a guess of mine)