Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}

If there is still a problem, Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Did this help?

While you are in safe mode;

Try disabling graphics hardware acceleration in Firefox. Since this feature was added to Firefox it has gradually improved but there are still a few glitches.

How to disable Hardware Acceleration {web link}

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

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

• https://support.mozilla.org/en-US/kb/font-size-and-zoom-increase-size-of-web-pages

If you have made changes to Advanced font size settings like increasing the minimum/default font size then try the Default font size setting "16" and the Minimum font size setting "none" in case the current setting is causing the problem.

• Options/Preferences -> General: Fonts & Colors -> Advanced -> Minimum Font Size (none)

Make sure you allow pages to choose their own fonts.

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

It is better not to increase the minimum font size, but to use an extension to set the default page zoom to prevent issues with text not getting displayed properly.