Open your HTML email signature file in your Firefox web browser.
Press Ctrl + A (Cmd + V for Mac) to select the entire email signature, then press Ctrl + C (Cmd + C for Mac) to copy it.
IMPORTANT: Do not use your mouse to select the area, only use Ctrl + A as you may miss out on grabbing some of the code.
Open Yahoo Mail in your Chrome web browser and click Settings at the top right of your screen. Select More Settings.
From the left sidebar, select Writing Email. You'll see your email address listed under Signature, enable the toggle then click inside of the text field that appears. Press Ctrl + V (Cmd + V for Mac) to paste your copied email signature.
Click Back to Inbox and compose a new email. Your email signature will now automatically insert when composing emails in Yahoo Mail from your Firefox browser.
Install the latest version of the Firefox Add-On here. Click Continue to Installation when prompted, then Add.
Open the Email Signature Rescue add-on by clicking the puzzle icon, then selecting Email Signature Rescue.
Click the Add New Email Signature button to create an install. Select your email client from the dropdown list, then paste your signature key into the next field and click Save Key.
Important: If you're using more than one email account, click Advanced Settings then enter the email address of the account that you want your email signature to work with. This will prevent the signature from being applied to all email accounts.
Compose a new email and your email signature will automatically insert into your emails.