The OSes that are used are 64 bits but the browsers are mostly (all?) 32bits on Windows, and probably 64bits on Linux.
Internet Explorer has 64bits builds on Win7, and Firefox has "almost" a 64bits browser on Windows too: Waterfox, which is a semi-official Firefox for 64bits Windows. Waterfox in particular claims huge improvements over base 32bits install, I would like to see how that translates into real-world.
Not sure about availability of 64bits editions of other browsers on Windows.
Here are my wishes:
-clearly mention if the 32bits or 64bits version of the browser is used
-where applicable and relevant, test with both 32bits and 64bits variants. I would like to see IE and FF split into 32 and 64 variants on Win for example.
I personally migrated from FF to WF on my machines 3 weeks ago and find it noticeably faster in everyday use. WF is now my main browser.