Get a wider screen !
 ;-)

Alternativly use the zoom feature but that can make text etc smaller and more dificult to read.

• keyboard shortcut Ctrl & + or Ctrl & - increase or decrease the zoom
  (Hold Control down press plus or Minus key)
• see how do i use zoom? <--- clickable link ---

It is possible to use extensions to adjust some of the magnifiction selectivly, but the zoom may be all you need.

If you have increased the minimum font size then try the default setting "none" as a (too) high value can cause issues.

• Tools > Options > Content : Fonts & Colors > Advanced > Minimum Font Size (none)

Make sure that you allow websites to choose their fonts.

• Tools > Options > Content : Fonts & Colors > Advanced > [X] "Allow pages to choose their own fonts, instead of my selections above"

• https://support.mozilla.org/kb/Changing+fonts+and+colors

Reset the page zoom on pages that cause problems.

• View > Zoom > Reset (Ctrl+0 (zero); Cmd+0 on Mac)

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

You can use an 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/