firefox persona header measurements (australis)
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must be 3000x200

cropped starting from top, so with only `tabs' and `navigation' toolsbars
enabled, 70 pixels from the top are shown (top to nav bar is 29 px)

cropped starting from right, so at the default width of 1280, the left 1720
pixels aren't shown. if the window is too wide, the accent colour is shown

a button placed at the far right of the tab bar will be 38 pixels (2972 from
left) wide (counting margins)

if it is transparent, the `Accent Color' is shown behind, but ugly seams start
appearing in the tab bar, so don't do it