I'd guess any site that uses a captcha. I've recently struggled with the same as OP. Captchas have become increasingly harder to solve, even when you solve it correctly, it still fails. I've noticed an uptick in "failed" captchas after switching to Firefox exclusively.
I sometimes get captchas on google(when researching and typing/modyfing queries too fast...), rateyourmusic, aliexpress, from the top of my head. hate those damn traffic lights!
