firefox python plugin

[M.N.A.Smadi]

hi;

i want to write a script to do the following:
when using firefox to download a textfile, instead of saving it to the 
hard dist, i want to run a python script on it, and i want the python 
script then to prompt me for the location where i want to store the 
processed downloaded file

any help is appreciated
thanks
m.smadi