Rich text editing
Some sites feature rich text editors that allow you to easily format (bold, italicize, etc.) text. However, some sites have not been coded to work with Firefox and Mozilla Suite's Midas feature; they only work in Internet Explorer.
Should you encounter one of these sites, the first thing you should do is to send an e-mail to the webmaster telling them of your problem. If enough people complain, the webmaster just might fix it.
For Firefox 1.0 - 2.0.0.x users, the eWebMail Plus! extension can provide rich text editing functionality for certain sites.
While you can use keyboard shortcuts (Ctrl+C or Ctrl+V) to copy and paste, some editors include toolbar buttons for this purpose and they don't work in Firefox or Mozilla Suite. Firefox and Mozilla Suite do not allow this for security reasons. If you had a credit card number, for example, copied to your clipboard and ready to be pasted, you would not want a webpage to use scripts to paste that information on the page and submit a form without your permission. If you understand the risks and still want to allow this feature, you can grant it access to the clipboard.
Known sites that don't work
Known sites that work
This article about Midas is a good place to start.