Neo, I am unable to duplicate your problem. When I go to Control Center > Appearance & Themes > Fonts > Window Title, change the font and click "Apply", the windows, including Firefox, change to my selected font. Did you click the "Apply" button after making changes?
Yes I did.
I actually did it twice to be sure, but no joy.
Here's another interesting problem.
The time displayed in VL is always wrong, even when it is correct in the BIOS.
I have it set for my time zone, always set date and time according to my time zone, and also set to always check the North American server, but it is always wrong.
The time problem isn't unique to Vector, other distro's have been experiencing the same issue.
The time difference is always exactly five hours earlier than it really is - exactly 5.
Now that we're past midnight, the displayed time is exactly seven hours later than it really is.