Firefox’s built-in PDF signing tool appears to apply automatic image cleanup/filtering to imported signature images. I tested this with multiple image formats, including PNG, SVG, JPEG, and others. The issue is not limited to transparent PNGs or alpha-channel handling.
The processing seems to merge, smooth, or remove small ink details. This may be useful for noisy scanned signatures, but it can damage signatures that are already prepared correctly.
In my case, the signature contains natural pen details, including line crossings, small ink variations, and dense stroke areas. Firefox’s automatic filter appears to blur or merge these details together. At stroke intersections and small ink drops, the result contains visible artifacts and looks less natural than the original image.
The main problem is that the user has no way to control this processing.
Expected behavior
Firefox should allow users to control how aggressively signature images are cleaned up before insertion into a PDF.
Ideally, the signature image import dialog should include an adjustment panel with:
- Preserve original image / no filtering
- Light cleanup
- Medium cleanup
- Strong cleanup
- A slider for cleanup/filter aggressiveness
- Live preview before inserting the signature
The most important option is Preserve original image, so users can insert the signature exactly as provided.
Actual behavior
Firefox applies automatic filtering to imported signature images. This can change the visual appearance of the signature, especially around:
- Pen stroke crossings
- Dense stroke intersections
- Small ink drops
- Thin anti-aliased lines
- Fine details in handwritten signatures
The final inserted signature may look worse than the original image.
Here are original image part and processed by filter after importing the image.
Suggested UX
When importing a signature image, Firefox could show a small panel like this:
Signature cleanup
[ ] Preserve original image without filtering
or:
Cleanup strength:
None — Light — Medium — Strong
with a slider.
A live preview would let the user immediately see whether the cleanup improves or damages the signature.
For images that are already clean, “None” should preserve the source image as closely as possible.
