This seems to be the case for most banks in Japan. Also a lot of government services. Last year I ended up posting some documents to immigration because I could not get their online form to work.
Most JP banks I’ve used work fine with any browser - I use my UFJ account regularly in Safari, etc.
Shinsei is just a crap bank, and IIRC Mizuho can be a PITA depending on what you’re trying to do.
I remember, for immigration process, having to use IE to download a PDF form and install Adobe Reader to open it. Strangely enough, it also had to be opened on a desktop machine, laptops are NG.
I am able to load Shinsei in Chrome earlier this year, was this a recent change? Do you mean banking or another service?
That is not true, I am able to use Shinsei on Mac with Firefox just fine.
Shinsei corporate account or personal? Different systems