Haven't seen it on FF here. on Mint 17.3 with FF44

Yep, also not having that problem here. Using FF 44 64-bit on a Win 7 computer.

Firefox 44.0 on Ubuntu 14.04 here, and everything looks OK.

No, I don't use any userscripts.

DIsabled all FF extensions, and opened GOG in private mode, still getting the same problem (see attachment).

What Firefox extensions do you have installed? If you're using NoScript, RequestPolicy, PrivacyBadger, Ghostery or other similar privacy/security addons, be sure to configure them to allow the various subdocuments and subresources on *.gog.com to work properly. Also, if you're using any addons that force HTTPS on everything, be sure to configure them to work properly for the various gog.com domains also, this can be a little tricky however for example HTTPS-Everywhere has a beta rule that is not present in the current version of that addon that works quite well compared to the default rule that is in the current stable release.

That's a couple of ideas/tips that might or might not help you but the image you posted suggests that possibly some subresources or CSS are not loading and addons/etc. are a common reason for that to happen on various websites.

Hope this helps.

Update: Looks like there were a few posts in between the time I hit reply, and the time I was able to post my above message. If you're not using addons and have restarted without them, then another possibility could be bad interaction with graphics acceleration. You might want to try disabling acceleration via about:config however I forget the exact key name so you might have to search the web for it.

Tried rolling back to FF 43 and FF 42, still have the issue. Rolling back to FF 41 is more complicated, since in that case I also need to roll back some other libraries, so I haven't tried it yet.
Tried enabling layers.acceleration.disabled and disabling layers.offmainthreadcomposition.enabled, didn't help. I couldn't find any other relevant settings by quick googling.

I got some jumbled page layout once when some scripts or css files from one of the CDN servers couldn't load. It was always the same server that failed - but it worked for other people. You could look the the console or network tab on the developer tools (F12) if something is not loading properly.

Not using any scripts or add-ons on my Chrome 48.0.2564.97 and I get this funny effect with your footer, when I move my cursor over the last post of a page -> http://imgur.com/OEYuaLt

This does look like Barefoot's Essentials "Collapsible Footer" option, along with the "Smaller Space before footer", but you said you don't use any scripts or add-ons. No idea then, other than double checking to make sure you don't have it installed by mistake.

I'm having the same problem right now. :-/

I can't say that this "just broke", but the new sort feature for achievements and controller support, etc... is not working correctly for all games. For instance I went to my library and sorted by achievements or controller support and it left out the Witcher 3 even though it's listed as having achievements and controller support and clearly does. Seems to work though if you sort by achievement's in the game section (the game is listed) but not from my library.

EDIT: It may or may not be specific to the Witcher 3. A quick overlook makes me think it's the only one but can't be sure.

I expect the reason is that Witcher 3's ID in one's library is not the same as its ID in the catalogue.