Hi mr_____t, if you check next to your question under Question Details, System Details, "More system details" you'll see that your Firefox provided this:

User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:47.0) Gecko/20100101 Firefox/47.0

Here's the first thing to check: How to reset the default user agent on Firefox. Does that work?

Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:47.0) Gecko/20100101 Firefox/47.0

That fact that it shows Ubuntu in the user-agent shows you may actually be using Firefox 47.0.x and not say the current 66.0.3 Release with a manually modified user-agent.

If you have Firefox 66.0.3 from mozilla.org installed make sure whatever method you are using to start Firefox, you are actually pointing to the 66.0.3 firefox .sh and not the package install of Firefox 47.0.x