The author of QiuReader has managed to make an exceptionally usable reader using existing libraries, with only a handful of minor issues.

It is light, fast, cosmetically nicer has intuitive controls and supports more expected functionality than "ePUB Reader" or "EPUBReader" (even allowing a mini library), while asking only for the most minimal permissions it can ( "Access browser tabs" ).

The performance and flexibility is largely is due to Futurepress' "epub.js" (https://github.com/futurepress/epub.js) , which does all of the heavy lifting.

Current issues as I see it are as follows:
- No page numbers.
- Changing settings or view format does not track your position (workaround this using bookmarks).
- The table of contents does nto track your progress
- Occasionally there seems to be lag in the page rendering, likely an epub.js issue.
- No RtL or vertical text support (the developer may be working on this but epub doesn't officially support it either).

I would like to see these things added in addition:
- TTS (text to speech) integration with the browser
- A document "map" alternative to the table of contents
- A navigable "progress bar" ala FBReader (could be an adoptation of the TOC)

notes:
- A user reported the TOC cannot be hidden, to do this, click the "hamburger" icon.
- I have had no issues with saved information or highlighting.
- I hope someone at Mozilla recognizes QiuReader for review as a Recommended Extension, it deserves that.