Tell them. I had a similar problem a couple of years ago, I logged a complaint and they eventually fixed it. It took a few months, IIRC, but they replied and confirmed that it was fixed.
They diagnose websites broken in Firefox and reach out to website developers, fix Firefox bugs, add Firefox UA overrides (see Firefox's about:compat page), and start discussions to resolve ambiguities in web standards. For example, what should happen for a broken image without an alt value?
it's related to adblocker, i eventually disabled adblock for that website, but without disabling it it wont work. i know it's not directly a firefox issue, but with mobile chrome not having adblocking it kinda is in that they don't care. it honestly took me to long to figure it because i could not believe that a multi national bank would have this broken website (and that is with new 2018-19 redesign). it's not the first website to not work without js, but you would assume that the bank website would be done in a way where if analytics is not loaded the rest of the webpage would continue to work.
