Some web mail clients (including Gmail, surpisingly) don’t allow you to set a rich text signature, this is, change the font type, color and size, and add images and links to provide additional contact information or promote something (like with a Firefox button).
With WiseStamp, a handy Firefox extension developed by WiseStamp, you can not only define fully formatted signatures, but it also eases the inclusion of instant messaging names (Yahoo!, GTalk, AIM, ICQ, Live, Skype and QQ) and social sites addresses (Facebook, YouTube, twitter, Flickr, LinkedIn, Friendfeed, digg, Yedda, Plaxo, Pownce, eBay, StumbleUpon, Upcoming, Blogger, Bebo): you enter your account name, and WiseStamp adds the corresponding service logo in front of it, and adds them all below your signature.
You can define create separate business and personal signatures and have them added automatically to supported Gmail, Yahoo! Mail, Hotmail and AOL Mail new messages. Or you can add it manually wherever you want by clicking on the status bar icon.
Get WiseStamp from Mozilla Add-ons.