I ❤️ the feature of Firefox called Reader View. I’ve been a fan for many years.
BUT, if you leave the popup in order to use the buttons while you read along, it blocks some of the text as the text automatically scrolls.
Because there's a lot of empty space on the side, could the popup start on the other side and appear over empty space rather than the text?
Here is for example how I implement this on my end:
/** for the "reader view" popup to NOT overlap the text **/
.dropdown .dropdown-popup {
inset-inline-start: -260px !important;
}