Firefox and Gmail: why is the page fit wider than my Pixel xl (7.1.2) screen?

[sfrrr]

I often read my Gmail in Firefox (53.0.2), rather than in the dedicated Gmail android app. But, I can't see the whole message. The right half is cut off. I even installed the Fit to Width add-on and it doesn't help. Doesn't happen if I open the same message in Chrome. How do I fix this?

Thanks.

 

[sfrrr]

Correction (couldn't find the Edit): why doesn't the message fit inside my Pixel xl screen?