[Bobalu]

Uninstall Adobe Reader and then install Foxit reader.
http://www.foxitsoftware.com/Secure_PDF_Reader/

[ican'tbelieveit]

http://helpx.adobe.com/acrobat/using/display-pdf-browser-acrobat-xi.html

https://support.mozilla.org/en-US/kb/view-pdf-files-firefox-without-downloading-them?s=View+PDFs+in+browser&r=2&e=es&as=s#w_check-firefox-settings

I don’t think I have the Adobe Reader plugin, so all of my PDFs open externally. But my son encountered this problem this week; PDF wouldn’t open in Waterfox. Think his resolution was to switch to different browser for that page, temporarily.

[JCBreckenridge]

Use a real browser. IE works just fine.

[Perseverando]

Here’s a discussion from several weeks ago: http://freerepublic.com/focus/f-bloggers/3042132/posts

[tomkat]

See #2 and just do it !

Adobe, notwithstanding Photoshop's wide audience, can pound sand.

Foxit Reader

[greedo]

Have you tried

about:config -> pdfjs.disabled   true

pdf.js viewer

[Gene Eric]

Try clicking the X to the right of the URL/address to cancel the download, then hitting the circular arrow to restart the download. Try that a couple of times. Once downloaded into the process space of FF, you should have the”Open With Different Viewer” button to the top-right of the document window in FF.

The bug (assuming it’s a bug) will no doubt be patched soon. I experienced the prob too!

[Doogle]

http://helpx.adobe.com/acrobat/kb/cant-view-pdf-web.html

[21stCenturion]

...

[ShadowAce]

[Straight Vermonter]

I see a lot of people have recommended Foxit. That’s a good add-on but if you update your FF it now has a built in PDF reader. I think everything after version 20 has this.

[Bloody Sam Roberts]

Dump Adobe and get Foxit Reader. It allows a lot more flexibility with PDF files and it's free. I've used it for years. It has a Typewriter tool that lets you add text to PDFs.

As for Firefox....I had just such a malfunction earlier this year and I tracked it down to a crappy update of Flash Player.

[JoeProBono]

[JoeProBono]