When I move back to a previous block using the keyboard, the cursor goes back, but I can't type right away.

Steps to reproduce the behavior

1. Posts → Add New
2. Write title
3. Write a first paragraph and press ENTER.
4. You are now in the second paragraph. Go back to the first paragraph with LEFT CURSOR key (or BACKSPACE).
5. You are now at the end of the first paragraph. Try to type something. Nothing happens. The cursor is there, but it is not responsive: you can only use keys like ENTER or CURSOR keys.

Notes

• It also happens with headings and quotes, not just paragraphs.
• It seems to work fine in Chrome.

Capture

Versions: WordPress 4.9.8 / Gutenberg 3.8.0
System: macOS 10.11.6
Browser: Firefox 62.0 (64-bit)