It's a common problem. What you get from the clipboard is not what you want.
When you copy some text from a web page or a Word document to the clipboard and then paste that text inside an email message or another rich text editor, the formatting (font sizes, colors, images) stays intact.
For Microsoft Word and other Office programs, you can use the "Paste Special" command that will auto-convert the pasted content from rich text as plain text format.
But if you copy-paste text between browsers, a neat solution is Clipboard Scrubber, which as the name suggest, automatically strips the text of all formatting as soon as you copy it to the clipboard. Everything that gets to clipboard after you hit Ctrl+C is converted to plain text.
You can also (optionally) remove all HTML tags from the clipboard text, which is excellent for copying and pasting things from the web, without getting all the extra markup.