[Affected versions]:
-66.0a1 (2019-01-07), 65.0b8, 64.0

[Affected platforms]:

• Windows 10,

[Steps to reproduce]:

1. Launch Firefox, open the devTools inspector;
2. Click on the 3pane toggle button;
3. Press the TAB key;

[Expected result]:

• tab option is marked as active;

[Actual result]:

• tab option is not marked with anything different

[Regression range]:

• not really a regression;
• this became an "issue" after a cleanup in bug 1476502;

[Additional notes]:

• Same issue encountered for the headers in the Network section;
• the close and show section(triangle) button are marked clearly when active (via hover, dotted line), but the headers not;

• see also bug 1466486#c15 the issue might be addressed already, but I was unable to find it or confirm yet;
• attached a recording with the issue, once the mouse cursor is moved from the section the user has no idea what is in focus.