change font on display

I just installed thunderbird on my new laptop. The windows font is set to times new roman. I would like to set my font in thunderbird to arial.this is for my viewing of everything on the screen. but everytime i go to options-display-formatting: i change the default setting "segoe ui" to "arial" nothing happens. the advanced font settings read: fonts for latin, proportional: sans serif, serif: cambria, san-serif:segoe ui, monospace: consolas. no matter how many times i change this & hit ok. it goes back to these same settings. what am i missing???