Hey, I tested this on two separate YouTube accounts with the latest version of firefox and I cannot find anything not working. I can switch between list and grid view without any problem. Dark and white mode.

I can only assume that this is a random temporary A/B test that YouTube is running quite often recently.

If you do not see the switcher and automatically land in the grid view, maybe the following could force the list view:
- Open the developer console (F12 or CTRL+SHIFT+i) on your keyboard
- Go to the console tab next to "Inspector"
- in the input at the the bottom put the following: "localStorage.setItem('yslv', 'list'); location.reload();"

If this wont resolve it or the A/B test will stay longer than 2 weeks I suggest to open an issue here with some screenshots: https://github.com/Flaflo/yslv-userscript/

Also as soon as this A/B test will get rolled out to me or the change will be permanent for all YouTube users I will surely fix this as soon as possible.