Unexpected style change bug

[Jab11n]

What happened?

Upon installation of the latest version of Waterfox on Windows 11, the tab bar uses the new style of tabs (rounded, floating) which is standard for the latest version of Firefox. I've experienced this issue twice where upon doing something Waterfox reverts the tab bar to the older Firefox style:

I've had this issue twice. The first time I changed the tab bar position to the bottom, and then this style change occurred. I refreshed Waterfox on about:support and the style changed back to the new Firefox style it had used originally. Upon opening the settings menu via the hamburger menu, without changing anything, the browser changed style again.

I'm not sure what the intended style is, because when I installed Waterfox on Linux Mint 22.1 the style it used right away was the older style I showed above.

When the style changed, it also changed the appearance of the shortcut icons on the new tab page from this:

(ignore the red line on the left side, I was measuring for this issue)
to this:

Reproducible?

• I have checked that this issue cannot be reproduced on Mozilla Firefox.

Version

G6

What platform are you seeing the problem on?

Windows

Relevant log output

[linkoviccsumoszky]

Same here + 6.5.5 breaks/ignores some of my userChrome.css customizations (such as the background color of the tab bar and the menu bar). It worked fine with previous versions.