Rated 5 out of 5 stars

le Reivax ar Aib. 6, 2007 · buan-nasc

You can set the default currency this way:
Open a blank page (type about:blank in the address bar).
Set the FROM and TO currency on the toolbar...
It will now be the default currency fo each new domain for which you did not chose another setting...
And, yes, even if you close and reopen Firefox...
Okay, you could not have guessed... ;-)