I plan to have a Firefox extension when I have time, I've done a rough
beta version here:-
http://www.businessinfo.co.uk/labs/ff_ext_beta_hackvertor/hackvertor.xpi
It's pretty much hacked together and the next version will probably
import XHTML namespaces for a better interface. When installed in FF
it appears as a red icon in the bottom right, you can select any text
in a web page and it will add it to the input section when opening. If
you don't select anything it will default to the current url.
I never really designed Hackvertor to be used in this way and I would
need to go through all the code and remove the string prototypes etc
in the final release but it gives you a chance to play within the XUL
environment.