This works for me:

Go to about:buildconfig, and look at the target. The first few numbers specify your build version.

For Example: in the attached screenshot, I have a 86/64 bit version of Firefox.

Thanks!

You will already have the answer from following iNerd's solution'

Note that on Windows you will normally be using 32bit versions of Firefox.

The 64bit versions for Windows are experimental and unsupported. You would be unlikely to accidentally use 64bit Firefox on Windows.

I have marked iNerd's answer as the thread solution.

Thanks, very much.

Your Firefox useragent also gives an idea of what you are using.

User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:19.0) Gecko/20100101 Firefox/19.0

The WOW64 part says is is 32-bit Firefox 19 running on 64-bit Windows 7 as the 64-bit Firefox would have Win64 in UA instead.