Right now, I am using pagezipper to auto-load the next chapter in the bottom of the page, ublock origin for cosmetic filtering to block unnecessary element of a web page from loading, and umatrix to not load things other than the necessary part of the story. Other than that, what can you guys recommend? If only there is a fullscreen mode for firefox, I'm content.
Full-screen you can access by F11 on the keyboard. If you want to force reader mode (F9 by default) where it doesn't show in the taskbar you can add this before the www or https about:reader?url= If you like having control over font and text colours better than through firefox's own options menu then try Dark Background and Light Text. Don't let the name mislead you, you can set either foreground or background colours however you like. Also if you don't mind slightly robotic voices with Read Aloud you can text-to-speech any text as simply as highlighting it, the advantage of this addon is that you can control voice pitch, speed, and volume (which reader mode does not support modifying) [sidenote: most of the voices are premium and require you to have a login or access to another voice platform like through amazon etc, Only Zira, Hazel, and David are the free default voices] Have fun.
For my dark mode needs I use Dark Reader on Firefox (both mobile and desktop). It may be redundant with that Dark Background and Light Text extension that @Lushwf linked though.