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}

This page has a code column and an output column. Where you have problems copying from? Also, are you using a built-in method such as

• select with the mouse
• Ctrl+c or right-click > Copy

or are you using a different method?

Are Copy and Paste in the right-click context menu grayed (disabled) ?

The left panel of the two seems to be a special editor (.monaco-editor) that comes with its own context menu with Copy and others.

• dom.event.contextmenu.enabled = true

Copy in this special context menu only works if this pref is enabled.

• dom.event.clipboardevents.enabled = true

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox