[Affected Versions]
Firefox 72.0a1(20191107094905)
Firefox 71.0b8(20191107101713)

[Preconditions]
Set 'devtools.netmonitor.features.webSockets' to "true" in about:config and restart firefox

[Steps to reproduce]

1. Launch Firefox
2. Press CTRL+L(CMD+L) keys, write http://janodvarko.cz/test/websockets/ and press Enter key
3. Press F12(Command Option K) to open Devtools
4. Go to the Network tab using TAB key
5. Navigate to "Connect" button from the page used on Step 2, using TAB key and press space when button is on focus.
6. Go to "Send" button using TAB key and press Enter key when button is on focus.
7. Using TAB key and ↓ key go to "http://echo.websocket.org/" websocket request
8. Navigate to "Messages" side panel using TAB key and →key

Actual result :
User is not able to navigate to "Messages" tab using keyboard.

Expected result:
User should be able to navigate to "Messages" tab using keyboard.

Thanks for the report!
Honza