This happens to me whenever I use the Galaxy Browser. First request is always "rate-limited". When I immediately reload the page, it goes through. I always thought it was a cheap tactic to have people install their app.
A refresh usually helps, and this "feature" isn't exclusive to Firefox.
For some reason, it is exclusive to the links opened from another apps and doesn't appear when accessing a link directly (via a refresh). That might narrow down your search a bit.
It happens to me often when I open a reddit link from twitter in an embedded WebView (using the bacon reader app) so it's unlikely to be a mozilla issue
I don't know why twitter blocks my Android Firefox, but I feel as if we both benefit.