How to Add an HTML Email Signature in Thunderbird

Open Account Settings from the menu and select an account. The checkbox sits beside the Signature text box on the main pane. Tick it before pasting markup, otherwise the code appears in your emails as plain text.

Paste the code, with 'Use HTML' ticked. This is the opposite of Gmail-style webmail editors, which want the visual result. Thunderbird interprets the markup at compose time, so what you paste is exactly what gets sent.

In Account Settings, enable 'Attach the signature from a file instead' and choose your .html file. Thunderbird reads the file fresh for each message, which makes it the easiest method to maintain across several accounts.

The image tags probably point at local file paths. Swap them for absolute HTTPS URLs on a public host, keep each file under 200KB, and the images travel correctly. Recipients may still need to click to load remote images the first time.

Yes, and it goes further than accounts. Every account has its own signature box, and the Manage Identities button lets you create extra sender identities under one account, each with a separate signature of its own.

No. Signatures live in the local Thunderbird profile, so each install needs its own setup. The practical workaround is the file method: keep signature.html in a synced folder and point Thunderbird at it on every machine.