With Safari or Chrome, I can select the dropdown (by clicking it or navigating there using the keyboard) and then enter the letter or number on the keyboard.
With Firefox, I have to click on the dropdown, scroll down looking for the right letter/number and then select it with the mouse. It takes at least 10 times as long as with other browsers. So I'm not using Firefox for that site.
Of all the interactive sites I use regularly about 10% have some sort of bug when visited with Firefox. And this is getting worse at an alarming rate.
I think many mainstream sites not created by actual tech companies have simply stopped testing with Firefox.
(I don't use ad blockers, just browser default settings)