Issue description

Pressing Backspace on empty lines has some weird behavior in Firefox.

Steps to reproduce

1. Insert a Paragraph block.
2. Type a single character.
3. Press Backspace.
4. Notice that the Paragraph now has 2 lines.
5. Type two characters in the first line.
6. Move focus to the second line.
7. Press Backspace again.
8. Notice that both the second line and the last character of the first line are removed.

This bug also occurs with other blocks that use RichText.

Testing environment

• OS: Antergos Linux
• Browser: Firefox Developer Edition 66.0b8
• WordPress: 5.0.3
• Gutenberg: 5.1 (the bug also occurred in 5.0... possibly earlier)