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Solution to zoom everything (was already previously posted):

Instead of using TF&SC (currently for TB 60 and lower), try this:
≡ > Preferences > Preferences > Advanced > General > Config Editor > I accept the risk! > search for "layout.css.devPixelsPerPx", modify value to "1.5" which is equal to 150%.

now close the settings and profit ;-)

Oceniono na 5 z 5 możliwych gwiazdek

Thank you very much Trenton!
Your workaround helps a lot to scale up the font size of plaintext e-mails.

@J. Winters you can even enter a value below 1 to scale down the font size (example: 0.5)

permalink of the workaround: