On my system, the side toolbar for the javascript PDF viewer had really small font for the different sections of a PDF. I am currently able to edit the font size and layout using userContent.css:
div.treeItem > a {
font-family: "Latin Modern Sans";
font-size: 16pt !important;
display: block ruby !important;
padding: 2px 0 10px !important;
}
However, this is rather hackish. It is possible to edit PDFjs using its javascript backend but I’m not sure how. It would be nice to allow change of the PDFjs UI font size in firefox.