All official Windows release versions of Firefox are 32 bit. Mozilla have been creating test versions of a 64 bit Windows version of Firefox but an official release is not likely any time soon. Mozilla developers also indicate that the 32 bit version is faster than the 64 bit test versions and recommend users including testers stick with the 32 bit version.

Waterfox is an unofficial 3rd party version of Firefox. If you use it be aware that quite a few plugins will not work as they are only 32 bit, and some extensions (especially those that have binary components) will not work.

I have used Waterfox under Windows 7 for about the past 4 months with no problems. I have not detected any virus or malware of any kind. I believe it is a good-faith project that does what it says, nothing more.

The speed seems to be somewhat better. One of the main advantages is that if I have a few hundred tabs open (which I do more often than you'd imagine), the 64-bit browser can allocate the RAM, whereas the 32-bit browser can not. In this condition, the 64-bit build suffers much less system slowdown (for a while), but eventually both will begin to bog down the system when a few dozen tabs are open and the browser is left running for a month or more. Occasional app restarts are still necessary due to severe memory leaks (regardless of build).

I notice absolutely no problems with any of my addons or plugins, which probably represents everything the average user could ever want.

There's little or no excuse to spread fear, uncertainty or doubt about using 64-bit Firefox, either compatibility or performance wise. If your plugins or addons are not listed, try it and see. They may work, have bugs, or not work, only one way to find out if it's right for you.

If you want to see performance differences objectively, without any political bias, do your own tests, with a stop watch, or with software profilers that can give microsecond or nanosecond accuracy of the time it takes to perform certain functions like page renders. I would assume differences of about 5% or less would be fairly negligible.

Plugins:

Adobe Flash (64-bit), Adobe Shockwave Flash (64-bit), Java JDK and JRE (64-bit), Microsoft Silverlight, VLC Web plugin

Addons:

AdBlock Plus, Add-on Compatibility Reporter, CookieEx Filter, Download Helper, eQuake Alert (menu bug, always transparent, happens in 32-bit Firefox also), FlashGot, Forecastfox, Form History Control, Jökulsárlón Download Manager, Live HTTP Headers, NoScript, Preserve Download Modification Timestamp, Session Manager, Tab Mix Plus, UPromise Turbo Saver, User Agent Switcher

There is a problem with "HP Smart Web Printing" addon, where by default it appears to be disabled with no option to enable it. It's a somewhat useless addon that is installed with the print driver without a choice. I can print just fine without the addon, even without the HP driver, as Windows 7 has a driver for my printer. But the HP driver does give a few extra controls at the OS level.

Edit 1: Lists of items, one per line, were collapsed to a paragraph, so I added commas.