Nope, kept Y pressed from the moment I touched the slider and it still overwrote the webkithax_tmp.bin. Btw, the one I downloaded from smea's website is 64kB whereas the one downloaded from BrowserHax is 32kB (that's how I know it's not the same^^). Don't know why though.
EDIT: don't know why but it finally worked (~20 attempts from this post) ! Not sure Y is for something because webkithax still hasn't been overwritten, but now I have menuhax so I should be fine.