Desktop mode browsing is now enabled by default for large devices. This can be controlled via a preference under Settings in "Site settings" previously named "Site permissions".

The problem is that I do not actually get the desktop site anymore, no matter how I set the preferences, or if the toggle in the 3 dot menu is on or off. I have the same sites open on my firefox desktop on Windows 11 and they look completely different to the sites on my firefox mobile on my android tablet now. The layout is completely different and considering everything is HUGE it is obviously a mobile site optimized for phones. The day before the update I got roughly the same site like I did on my desktop firefox.

It happens on all sites I visit frequently, like forums, shops, etc. so it is not a server side change.

edit: Since you write "... for large devices ..." I wonder if for some reason my device is not recognized as "large device" and therefor doesn't get the desktop site. Where does this constraint come from? The setting in the preferences is just called "always get desktop site", there is no word about it being only for large devices.

I've just tested it again: I deactivate the new preference "always get desktop site", visit some site and get the mobile site accordingly. Now if I click on the toggle in the 3 dot menu, the site is reloaded but I get the mobile site again. If I click on the 3 dot menu, the toggle for "load desktop site" is deactivated again.

Another update: There definitely is some bug involved. If I deactivate the "always load desktop site" setting in the preferences and also deactivate the "desktop site" toggle in the 3 dot menu, I am able to get the desktop site on most sites, after some reloading and/or tab switching. On the other hand, if I activate the "desktop site" toggle in the 3 dot menu again, firefox loads the mobile site instead...

Where is the best place to report this?

You can report it on https://bugzilla.mozilla.org. Please also share your bug number here.