EDIT:
Hello Mamroth, I have done a deep test. I likely have recreated the same problem with the extension not loading properly on high-load situations.

I advise you to use the Beta version for now as that passed all the deep test with no clashing for me on various extensions whatsoever. I will be rolling the fix to the normal version soon.
(https://addons.mozilla.org/ja/firefox/addon/soba-youtube-side-comments-%CE%B2/)

Regarding the ""Dark background and Light text"
Is it possible that you might be using any Dark Mode Extensions? While the Beta worked for me even with the Dark Reader extension enabled, if you still see issues, try disabling any Dark Mode extensions and sticking to Firefox’s native Dark Mode.

I'm on Firefox Stable 146.0.1 (Win11). If the Beta doesn't solve it for you, please email me at reinaro076@gmail.com with your setup details (browser version, other active extensions) and I'll take a closer look.

Thanks
-renaro