Although it is not always reproducible, but currently the main navigation menu (orange one) is very wide, which makes the screen go far to the right on mobile and this can create problems with centering dialog, like login or message input. Ideally on mobile this menu should collapse into a hamburger button. Or maybe there can be at least a quick fix to avoid such issues on Firefox Mobile.
I defer to Flow on discourse issues.
The navigation bar is part of website itself. Although maybe it is possible to do something about Discourse to prevent that.