This is an awesome, awesome little app!
Valve should so implement this into Steam themselves, some really helpful stuff it shows you. Stuff which just seems so obvious really, like your total transactions amount, great idea and the price history bit is excellent. Thanks for posting this OP!
Also, to clear up any misunderstandings - it works both in Firefox and Chrome, however it seems only the 'full' features work in Chrome like the your transaction totals and game price history. Also the 'options' in the little top drop down menu only works in Chrome. In Firefox, it does show your total transactions but you cant seem to change the currency from $ or do much else.
So, basically its not very good in Firefox (maybe its still work in progress) but in Chrome it works well.
EDIT: Also, remember you have to login in Chrome or Firefox for it to work and the 'Install Steam' button at the top remains, even after you have logged in but there is an option in both Chrome (top drop down menu) and Firefox (Add-on options, under Tools in menu bar) to hide the 'install steam' button.
Christ, Ive spent £605 in the Steam Store. Im guessing it does not count games bought on disc or elsewhere like GMG and then activated on steam, only Store transactions.