More debugging information. This is from Firefox 137.0.2-1 as installed on my linux (Debian, PopOS) laptop.
First, I tried disabling cache and clearing site data, but unlike @Anaxim's experience this didn't help on my instance.
Second, I wanted to clarify that the problem with the manga reader on Firefox is strictly limited to loading the chapter. Once loaded, I can move from page to page without problems. That is, I can turn on privacy.resistFingerPrinting once the chapter is loaded and the reader works fine going from page to page.
Third, by switching the privacy.resistFingerPrinting on and of during load, I've isolated this error:
Uncaught Error: Permission denied to access property "type"
NextJS 5
Learn more
framework-717b5372727d76c7.js:25:92626
NextJS 4
forEach self-hosted:157
D NextJS
sendSyncMessage moz-extension://e1185278-3333-41bb-aefd-8b59cabfee08/nscl/common/SyncMessage.js:564
refetch moz-extension://e1185278-3333-41bb-aefd-8b59cabfee08/content/staticNS.js:126
fetchLikeNoTomorrow moz-extension://e1185278-3333-41bb-aefd-8b59cabfee08/content/staticNS.js:144
fetchPolicy moz-extension://e1185278-3333-41bb-aefd-8b59cabfee08/content/staticNS.js:95
fetchPolicy moz-extension://e1185278-3333-41bb-aefd-8b59cabfee08/content/staticNS.js:85>
The "learn more" link says, among other things, that :
"There was attempt to access an object for which you have no permission. This is likely an <iframe> element loaded from a different domain for which you violated the same-origin policy."
Thanks again, @chocolatkey , for your efforts. Hope this helps...