When I updated to MacOS Tahoe 26.0, Firefox prompted me to autofill security codes received by iMessage. That happened once or twice and then stopped working. I just tried it in Safari, and it does work. So, something is wrong in the handshake between Firefox for Mac 144.0.2 and MacOS iMessage. Here is how to replicate it.

Login to a website that requires two-factor authentication. Have a security code texted to your iPhone. When that text appears in iMessages on a Mac linked to the iPhone, there should be a small popup asking if you want to insert the security code into the web page. In Safari, there is. In Firefox, there isn't. This feature appears to be broken in Firefox.