Firefox's PDF viewer incorrectly applies user-defined font settings intended for HTML pages, resulting in illegible PDFs and requiring time-consuming workarounds. Key Points: 1. When "Allow pages to choose their own fonts" is disabled in Firefox settings, the built-in PDF viewer renders some PDFs with incorrect fonts, replacing characters with icons and gifs. 2. This setting should only affect HTML pages, not PDFs, as PDFs are designed to maintain the author's exact formatting. 3. Current workarounds (saving/opening PDFs locally or toggling the font settings) are inconvenient and disrupt workflow. (Open a settings tab, navigate to fonts, navigate to advanced, toggle the option, select okay, return to prior tab, reload, view without error, then repeat the process to restore preferred settings.) 4. Users who prefer custom fonts for HTML content are forced to choose between readable PDFs and their preferred web browsing experience. Background: HTML is intended to decompose information exchange in such a way that the viewer has control of aspects such as fonts; while PDF is a format designed specifically to present exactly as the author intended. The difference is the primary reason an author would choose to use pdf vs html. The issues can be reproduced with B612 and other fonts, in Firefox 127.0.2 and many earlier versions, on github repositories pages that embed pdf files, and most other pdf files opened with the viewer. Proposed Solution: Implement a separate font control setting for the PDF viewer, allowing users to maintain their custom font preferences for HTML content while correctly rendering PDFs with the author's intended fonts. Or, simply disallow the setting to apply to the PDF Viewer. Impact: Resolving this issue will significantly improve user experience, maintain the integrity of PDF documents, and eliminate the need for disruptive workarounds, ultimately enhancing productivity for affected users.