In menu-cusomize, check title bar. It will partially fix it but the tab bar is still not glassed

(I always used Firefox with title bar on, whether there is a bug or not)

It probably happens because Firefox's CSS doesn't have support for Vista as is wasn't ever intended to be launched on it, and it simply counts Vista as 10 because it's not Win 7 and 8.

You'll need to tinker with CSS a little... What I think can be done is go to
https://github.com/Feodor2/Mypal68/blob/main/browser/themes/windows/browser.css
https://github.com/Feodor2/Mypal68/blob/main/browser/themes/windows/browser-aero.css
https://github.com/Feodor2/Mypal68/blob/main/browser/themes/windows/compacttheme.css
these files (I might miss some but these are main ones),

find Windows 7 @media in Mozilla's CSS files (sth like this: @media (-moz-platform: windows-win7) ) and add windows-vista; before so the thing will look sth like @media (-moz-platform: windows-vista; windows-win7).

_
Alternatively, you can try to "fix" it without recompiling using userChrome.css.
r/firefoxCSS wiki on Reddit is a great place to learn the basics. You'll have to copy all CSS aforementioned rules from the CSS files above and replace windows-win7 with windows-vista. This should probably work too

find Windows 7 @media in Mozilla's CSS files (sth like this: @media -moz-platform: windows-win7) ) and add windows-vista; before so the thing will look sth like @media (-moz-platform: windows-vista; windows-win7)

nvm, I did a few experiments and I see the issue now. Mozilla deleted "windows-vista" and "windows-xp" media queries in FF57, so the method above won't work.

The solution I came up with is to use -moz-windows-glass, which is only triggered by Windows Vista and 7, when Aero Glass is enabled. Pay attention this method will not work if you're using Vista with Aero Basic theme (which still looks pretty good).

You can get modified CSS files here:
https://github.com/1280px/Mypal68_AeroVista/commits/main

Also I made a pull request (tested on Windows XP, Windows Vista and Windows 7):
#63

I shall look into this after resolving major issues like crashes.

I suggest that this issue is to be closed because it is solved by #63. Thank you, @1280px!