Also annoying with T-Mobile, their “post-paid” account site seems to be pretty broken in Firefox. I had managed fine with Firefox on the site previously, but haven’t had to go in there for quite a while since I have auto-pay set up. The “pre-paid” site has continued to work, though with some issues. For my post-paid account, I had to use Safari, which seemed to work just fine. I guess this is another example of the lack of attention the least used of the big three browser engines gets. With less than 3% market share (via CanIUse), developers even at big companies seem to ignore it sometimes.
The lack of browser engine diversity in the ecosystem means the big ones and their companies, namely Google and Apple, but mostly Google, can control the direction of the web.
I develop in Firefox unless I hear of an issue in another browser, so Cogneato and other sites I work on will definitely support it.