As you say it is new = not a high likelihood of multiple responses from people who have grown it for an extended period. Otherwise:

'Firefly Sunshine' - Yarrow - Achillea hybrid Awards | Proven Winners

Achillea 'Firefly Sunshine' from Penn State Trial Gardens (psu.edu)

USPP32403P2 - Achillea plant named ‘Firefly Sunshine’ - Google Patents

The Penn State trial results are helpful. Thanks!

Bluestone is holding 50/50 Fridays sales again this spring and I just ordered 3 Firefly Sunshine yarrows at half price. Yay!

Yay, cecily! Please post pics and let us know how they’re doing this summer; I purchased Firefly Peach Sky last year and am excited to see them emerge and bloom this year. Just a heads up, though….groundhogs find them delectable and I haven’t yet found a groundhog repellent that works effectively.

July 1 update: they've been in the garden for three months and in bloom since early May. The color is a lovely clear lemon. So far so good:)

I'm sold, that's a beauty both in bloom and foliage and having a lengthy flowering time, will invite some home to my garden asap! :)

Oooh, I have been resurrecting an interest in achilleas too. I gave up on most of them as they all seemed to stretch to ridiculous heights...then fall over. I now want mannerly plants which top out at 60cm or so. Not sure Firefly varieties will be available in the UK though, but yours looks lovely, Cecily.

My brain thinks yarrow needs a hot, dry, full sun site so I've never tried it in part shade. Perhaps others know better than I ;)

The filipendula (Coronation Gold) with green foliage manages in part shade, ime, but the ashy leaved yarrow really do need full sun or it just stretches for the light and inevitably, falls over.