Adobe Acrobat has a new security hole found every day (OK, slight exaggeration), and is slow and bloated. The appeal of a light-weight in-browser PDF viewer is undeniable. Now both Chrome and Firefox offer this option. But the Firefox viewer produces blurry text, and I do not recommend for anything but quick on-screen previewing.
The Firefox PDF viewer does not support "sub-pixel" font rendering which means that any text smaller than about 16 points looks blurry and is hard to read. If your only goal is to print the PDF after checking that you downloaded the right one, it's certainly good enough, but if you want to save a tree, you'll soon get tired of reading such blurry text, unless you like to read each page zoomed in so that only 1/2 of it fits on-screen at a time.
I'd provide a screen shot but "sub-pixel" rendering means splitting each pixel into its red/green/blue components and turning on more of them or less depending on the width and locating of the stroke that passes through that pixel. So it only looks good if you know what order the colors are arranged on your LCD (RGB, GBR, etc) and if you guess wrong it actually looks much worse. So take it from me.
Or, if you want to see yourself, here's the best step-by-step instructions I've found:
After following those steps I still needed to go into the browser preferences and switch the content handler for PDFs from external to "Firefox". Perhaps that is all you really need to do, I don't know.
Chrome supports sub-pixel rendering, and I find I can read an 8x11 page full-screen on my 24inch LCD with no problem. The same is true with Acrobat, of course, but Adobe's product is just too unsafe to use. The only time I've ever gotten anything like a virus was from a hole in Acrobat (that has happened twice to me, in over 20 years of computer use, meanwhile obsessive virus scanning has NEVER turned up a single virus on any file I've downloaded from the Internet).