Hi gentlemen,
This display problem happens because the Inoreader website does something called
user agent string sniffing. This means that the website checks if you're using a computer or a mobile device and it offers you the proper version: full [for computers] or mobile [for smartphones and tablets] by parsing every web browser' user agent, no matter if it's a desktop/laptop browser or a phone/tablet.
For instance, this is the Pale Moon 26 x64 user agent string on Windows 10 64 bits, with Firefox compatibility mode off:
Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:2.0) Gecko/20100101 Goanna/20160121 PaleMoon/26.0.0
And this is the Pale Moon 26 x64 user agent string on Windows 10 64 bits, with Firefox compatibility mode on:
Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:38.9) Gecko/20100101 Goanna/2.0 Firefox/38.9 PaleMoon/26.0.0
However, there's a problem here. The Inoreader user agent database probably doesn't check for the Pale Moon web browser itself, but for "main" web browsers only: Mozilla Firefox, Internet Explorer, Safari/Chrome/Opera/Edge, etc. So until Pale Moon's previous version [25.8.1] Inoreader recognized Pale Moon as Firefox, because up to Pale Moon 25.8.1 its user agent was similar to the Firefox's. Beginning with Pale Moon 26, there are some modifications in this web browser's UA string —in order to make a difference with Firefox—. With the Firefox compatibility mode on, Pale Moon "masks" itself as Firefox 38, so it should work with Inoreader. However, Pale Moon 26's user agent also includes
Goanna/20160121 or
Goanna/2.0 —
Goanna is the new render engine for Pale Moon, instead of the original Gecko from Mozilla—. And here's this display problem creator: there's a mobile device called
Wiko Goa, which UA string looks like this:
Mozilla/5.0 (Linux; Android 4.4.2; GOA Build/KOT49H) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/39.0.2171.93 Mobile Safari/537.36
So, the Inoreader website —between many others— detects
Goa in the term
Goanna, and "thinks" Pale Moon is a Wiko Goa device, so it offers you the mobile version.

But we all know that's wrong!

Pale Moon —whatever its user agent string is— is totally capable of working with computer/full versions of websites, as capable as Firefox is. So, when you face a situation like this, it would be helpful if you contact the site's administrator/webmaster in question and ask him/her to stop doing this user agent string sniffing stuff, or at least add correct recognition of Pale Moon.
As you can read here,
you're not the only one who have this issue.
If you really need to use Inoreader with urgency,
there's a workaround to back to Inoreader's full version in Pale Moon. If you need help setting this, let me know and I will tell you how to do this.
Best to you both!