Firefox PDF Viewer Has No Dark Mode
Firefox's built-in PDF.js viewer lets you read PDFs without a plugin, but as of 2025 it still has no dark mode toggle — so a PDF snaps from your dark browser UI to a blinding white page.
Why there is no toggle
A real dark mode needs either a crude CSS inversion filter (which also inverts images) or full rewriting of the PDF color operators (which means integrating a library like MuPDF into the render pipeline). Neither is trivial to get right for all PDFs, and Mozilla has not prioritized it — there is no announced ETA.
Workarounds and their limits
- Forced Colors / High Contrast mode — Firefox inherits the OS high-contrast theme, but the result is monochrome, not dark mode, and replaces diagram colors with system accent colors.
- Dark Reader extension — works on text-heavy PDFs in a browser tab, but its inversion makes photos and charts look like negatives.
The clean fix: convert before you open
Convert the PDF to dark mode first and Firefox renders it dark with no extensions or workarounds. DimlyPDF does it in about five seconds in your browser; because it only rewrites color operators with MuPDF, you can still select text, search with Ctrl+F, copy content, and follow hyperlinks in the converted file.