That's because of a problem with the website's script. It performs some browser sniffing for loading the script on the website. The easiest solution to this problem, which wouldn't exist in the first place if FA were made by competent people, is to right-click on the page, select 'Edit Site Preferences', and then set 'Browser identification' to 'Identify as Firefox', under the Network tab.

(I use Opera too, you see.)

Thank you. I'll try it if problems still happen the next time I update.