Post text box doesn't load on the Qwant browser on Android, it also doesn't load on the Fennec F-Droid browser either, but the Lightning browser, which uses the system webview works fine.
However on my Galaxy S7, running the same custom rom and Android version as my main phone (CrDroid 5), MSFN works with Fennec F-Droid, but it causes Lightning to crash and has a 50% chance of closing all my tabs.
However, updating the version of Fennec F-Droid on my main phone fixed the issue, but my S7 is running an old 68.x release instead of the latest 84.1.2 now on my main phone.
I also tested a few older iOS devices I had laying around, and it's much easier to test iOS, because it's either bundled WebKit or die.
iOS 12.1.4 on a iPhone 5s worked, but the post box does not show any text that gets typed into it.
iOS 9.3.6 on a iPhone 4s worked, but the post box doesn't show up
iOS 6.1.3 on a iPhone 4 worked, but the style formatting was broken, and post box again didn't show up.