Update 4-Sep-09: An employee from Skype emailed me and told me that they're released a new version of Skype (version 4.1 Hotfix 2) that solves the problems below. You can download it from http://www.skype.com/go/download.
We've recently had some users reporting problems sending email in Firefox via the web interface. After a bit of investigation, it seems the problem is related to a recent update to Skype and the Skype Firefox extension.
By default when you install Skype, it installs an extension into Firefox which tries to find all phone numbers on a web page and allow you to easily make a Skype call to that number. Unfortunately the latest version of that extension seems to be quite buggy and destroys basic Firefox functionality. You can read about the issues here:
We recommend users using Firefox who have Skype installed on their machine disable the Firefox Skype extension until this is fixed. You can disable the extension by starting Firefox, going to Tools -> Add-ons, finding the Skype extension, and then either clicking Disable or Uninstall. You'll have to restart Firefox after this change.