FF89: Tabs have unnecessary space above them when window is maximized

[epitaque]

Sanity checks (must complete)

• I have read and followed the installation instructions in the README
• I have not modified the userChrome.css file
• I have tested the latest release for my Firefox version, or commit on master/beta branch (beta is for Firefox Beta only)

Describe the bug
In past releases, there was no space above tabs when the window was maximized, they went all the way to the top of the screen. Now there's a gap between the top of the tab and the top of the screen.

To Reproduce

1. Open FF89 with this modification on Windows 10 and maximize the window.

Screenshots
How tabs in maximized windows in FF89 currently look (notice space above the tab):

How the tabs should look (and how they currently look in Chrome):

System info

• OS: Windows 10
• Firefox version: 89

[fmeyertoens]

Do you have browser.tabs.extraDragSpace enabled in about:config by any chance?

[Yumega]

Do you have browser.tabs.extraDragSpace enabled in about:config by any chance?

Do I have to creat browser.tabs.extraDragSpace? It doesn't have this

[Yumega]

Do you have browser.tabs.extraDragSpace enabled in about:config by any chance?

I created it and make it false, but it still have space above the tab

[epitaque]

Do you have browser.tabs.extraDragSpace enabled in about:config by any chance?

I did have it enabled. I just disabled it, then restarted Firefox, and it still looks the same.

[Yumega]

The toolbar of Chrome looks more compact, this theme is still a bit bigger, how to make it same size as chrome?

[jonas-roeser]

I've encountered the same issue in both the 89.0 and 88.0 releases. In 80.0 it was still fine.