What the second tweet means
1. Chrome done screwed up.
2. Devs develop a website assuming Chrome is right and is the standard.
3. Website does something different on Firefox.
4. People assume Firefox is wrong.
5. Devs may or may not fix the website bug.
If they don't and the website is popular:
6. Firefox changed to ape Chrome behaviour.
This is why browser monoculture is a bad thing.