I've noticed gmail randomly blocks Firefox these days under the pretence of "your browser may not be secure" (i.e it doesn't persist through page refreshes), similar to how they try to make you do a captcha unless you refresh the page... I seem to have less and less control over where and how I am allowed to sign in (even thought I'm using a U2F key), and as a result I'm definitely getting pushed closer to the thresh…
Your device is probably pwned. But go ahead and blame Google. I love how people just ignore these warnings.
It has nothing at all to do with how secure the browser on the end user's computer actually is.