Gmail supports signature but it won’t allow you to use multiple signatures. Blank Canvas Gmail signatures is a Firefox addon that allows you to create four HTML signatures for any Gmail account.
For creating signature you can use HTML and inline CSS, remember mail programs may not support external CSS so be sure to use only inline CSS.
The extension is an experimental addon and it is not perfect while using you will notice that sometime it hangs up or it won’t allow you to edit. To download the plugin you need to have an account with Mozilla addons site because this is an experimental addon and Mozilla has not yet approved it. This is a great addon if you prefer to use multiple signatures in Gmail and useful if you have multiple Gmail accounts. [Via Webware]