Any log I can check, Perturbatio?
Unfortunately, no, I can't write to anything outside the config using a greasemonkey script.
Something you can try checking:
select all the page content (CTRL+A)
right click and view the selection source
then do CTRL+F and search for yui.yahooapis.com
if that's there, then the script is at least getting initialized.
I wonder if FF3 has issues with referring to external JS? It could break a lot of sites if that's the case.