Your zoom controls.

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

Security Issue: Update your Flash Player Version 19.0.0.245
https://www.adobe.com/products/flashplayer/distribution3.html Note: Windows users should download the ‘’’active_x’’’ for “Internet Explorer,” and the ‘’’plugin’’’ for “Plugin-based browsers” (like Firefox).

Note: Edge on Win10 does not use ActiveX version but its own Flash.

See if there are updates for your graphics drivers https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

I tried that, but it doesn't solve my problem. Either my letters are too small and the page fits the screen, or my letters are ok, but the page doesn't fit the screen so I have to move it right and left all the time

Try to set layout.css.devPixelsPerPx to 1.0 (default is -1) on the about:config page. If necessary adjust its value in 0.1 or 0.05 steps (1.1 or 0.9) until icons or text looks right. Make sure to check the value you enter before clicking OK.

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config

You can look at the Default FullZoom Level or NoSquint extension to set a default font size and page zoom on web pages.

• Default FullZoom Level: https://addons.mozilla.org/firefox/addon/default-fullzoom-level/
• NoSquint: https://addons.mozilla.org/firefox/addon/nosquint/

This only changes the size of the web page and the toolbar, but the letters on web pages are still too small in comparison to the size of the page

Here are the images. In the 1st one the web page is too big, but the letters are ok and in the 2nd one the page is ok, but the letters are too small

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac Options) key, and then starting Firefox. Is the problem still there?

Yes, it is, the letters are still very small

You may have zoomed web page(s) by accident. Reset the page zoom on pages that cause problems.

• View > Zoom > Reset (Ctrl/Command+0 (zero))

• http://kb.mozillazine.org/Zoom_text_of_web_pages

You can look at the Default FullZoom Level or NoSquint extension to set a default font size and page zoom on web pages.

• Default FullZoom Level: https://addons.mozilla.org/firefox/addon/default-fullzoom-level/
• NoSquint: https://addons.mozilla.org/firefox/addon/nosquint/

You can rename (or delete) the content-prefs.sqlite file (content-prefs.sqlite.old) in the Firefox profile folder to reset website specific data.

You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Using the extensions, I was able to enlarge the letters, but everything is still disproportional on every page I visit. For example, I can't see the whole picture on my e-mail (see image) - the left side is missing. For now nothing seems to work... It was like that when I installed Mozilla

Start your Computer in safe mode with networking. Then start Firefox. Try Safe web sites. Is the problem still there?

Starting The Computer In Safe Mode;
Free Online Encyclopedia

Check that you do not run Firefox in compatibility mode. You can open the Properties of the Firefox desktop shortcut via the right-click context menu and check the "Compatibility" tab. Make sure that all items are deselected in the "Compatibility" tab of the Properties window.

It seems that the problem was in the drivers for my graphic card. Thank you so much for trying to help, I appreciate it a lot!

That was very good work. Well Done.