Appearance > Customize freezes when open (9 out of 10 times) - not usable

It's not often i hear that. I suppose its time to disable some extensions to see which one is not playing nicely.

As it turns out, Dashlane is the root of all evil. Very odd.

But thanks, I think this solved the issue, or at least I found the problem.

Glad you found the solution. Could be a cookie issue. Not sure if using incognito browser window would resolve.