All platforms with Firefox: If you’re constantly typing “Thanks for writing” at the end of all your web email messages, the Paste Email Firefox extension can insert it—and any other repetitive text phrases—with the click of the context menu. Based on the same concept as our home-built Texter and Mac-only TextExpander, Paste Email saves you repetitive typing by offering a selection of pre-entered phrases (like your email address, a link to your web site FAQ, a signature). Unlike global text replacement applications, Paste Email is browser-specific; it also only does single lines instead of text boxes.
Its more fully-featured sibling, Paste Email Plus, can insert more than one line of text, but it’s still listed in the experimental section of Mozilla Add-ons (so try it with caution). The previously mentioned Signature extension does the same job, but isn’t yet Firefox 3 compatible. Paste Email is a free download, works wherever Firefox does.