Check your zoom controls.

<Control> + (plus) enlarge <Control> - (minus) reduce <Control> 0 (zero) restore

NoSquint {web link} NoSquint allows you to adjust the text-only and full-page (both text and images) zoom levels as well as color settings both globally (for all sites) and per site.

NoSquint doesn’t work with Firefox v47+ Please try NoSquint Plus {web link} Manage site-specific zoom levels and color settings

I'd like to hear from real specialists, from a real Mozilla employee, who understands the gravity of the issue and isn't ready to indicate improvised solutions.

Hi,

The people who answer questions here, for the most part, are other Firefox users volunteering their time (like me), not Mozilla employees or Firefox developers.

If you want to leave feedback for Firefox developers, you can go to the Firefox Help menu and select Submit Feedback... or use this link. Your feedback gets collected at http://input.mozilla.org/, where a team of people read it and gather data about the most common issues.

sorin86 said

I'd like to hear from real specialists, from a real Mozilla employee, who understands the gravity of the issue and isn't ready to indicate improvised solutions.

FredMcD took the time to give you the   perfect   solution,   but if you're not happy with that,  then I suggest you write a letter to Mozilla headquarters.

(looks like FredMcD and I were posting a reply simultaneously)