Paste Control od Gateron

Paste Control gives you a simple per-site switch for pasting, right from the toolbar.

What it does:

Block paste — stop pasting on a site (Ctrl/Cmd+V, and paste via input events).

Force paste — re-enable pasting on sites that deliberately block it, such as some login and banking pages.

Default — leave the site alone and use the browser's normal behavior.

Subdomains:
A rule applies to a site and all of its subdomains, so a rule on example.com also covers www.example.com and shop.example.com. You can set a separate rule on a specific subdomain to override the parent.

Managing your rules:
The options page lets you add sites manually, see every rule in one list, remove them, and import or export your rules as a JSON backup. Your rules also sync across the Firefox browsers you are signed into.

Privacy:
Paste Control makes no network requests and collects no data. Everything is stored in your own browser.