Originally Posted by Lord Venom
Well until the release of Windows 7, 32-bit operating systems were still the most used out there. Since 64-bit has caught on it's only a matter of time people stop using 32-bit browsers in favor of 64-bit. But that'll take time, a few more years at least. It's a good start to have 64-bit Java, Flash and Silverlight plugins but there's many, many to go that's needed like QuickTime, Windows Media and Battlelog for Battlefield 3.
And yes, a proper PDF Acrobat plugin from Adobe. Though if you can wait for Firefox 16, there will be inline PDF support which should work in Waterfox too.
All this is not proper to browsers. There is the same pb with other programs : no 64 bits version of LibreOffice (for example) is previewed. A 64 bits version of MSOffice 2010 does exist but it is not recommended by the authors.
Firefox 15 will contain a build-in and free PDF reader, formerly PDF.js extension, and will no longer need an Adobe reader/acrobat plugin.
As I said few posts above, a 64 bits browser should not depend on 64 bits plugins : Opera solved the pb with oopp (out of process plugins) method.