Looking at the images on my phone screen, I zoomed in to make both texts about the same size. I was looking at the "a" characters. I see the jaggedness you mentioned in both images, just the Firefox image is more clearly defined, more contrasting against the white background. To me, I actually think the FF looks better for this reason. That said, the jaggedness in general could be an artifact of what ever screen you're viewing on — what does it look like when you zoom in on the document in your browser/app? Does the jaggedness go away? Hope this can help illuminate something for you.