We have many many users editing Typo3 using Firefox and a number have come across a security feature in Firefox that acts like a bug in Typo3.  

If you use the copy, cut, paste from the action icons above the editor:

Or if you use the contextual menu

Or if you copy something in MS Word

And then try to paste in using either the paste action icon in the RTE or the right click paste.  You will get the following:

This isn't Typo3 related at all.  It's a security feature in Firefox.  There isn't a ready component for you to install, so I would recommend that you not click "OK" but you are welcome to try (and if you find one that works please let me know!).  

The workaround this is to use keyboard shortcuts. 

Windows:

• Ctrl + C = copies the highlighted text
• Ctrl + X = cuts the highlighted text
• Ctrl + V = pastes the clipboard contents

Mac

• Command + C= Copies the highlighted text
• Command + X=cuts the highlighted text
• Command + V=pastes the clipboard contents

For information from Firefox itself please go to: http://support.mozilla.com/en-US/kb/Granting+JavaScript+access+to+the+clipboard