What really chaps my ass is when they don't bother to tell you. It always happens when I'm filling out a form and find out that the submit button just doesn't do anything. Then I have to go back through chrome just to fill out the same form a second time.
Worst part is that I get this with the government website in the UK. For me it was a sub menu which was supposed to appear when a certain option was clicked. It wouldn't display the sub menu in Firefox. Had to redo the damn thing in Edge...
I just call places like that and tell the service rep the website didn't work. They do it for me on their end usually and it costs the company more money for the trouble.