Do you know the page that does not allow you to paste your e-mail to the confirmation field?
Now impots.gouv.fr goes a step further and does not allow you to copy from the first field.
Edit: managed to beat it with the Firefox Translation add-on by trying to “translate” my e-mail from the context menu. One step less-worse than searching for it I guess…
Also it's one JS function, so while there might be a Monkey script option I wouldn't use it filing taxes.