This extension provides for all your conversion desires. The primary focuses being:

- Intelligent conversion and detection of currencies

- Automatic and non-intrusive in-site conversions

- Always access to converter no matter where you are

- Simple UI, yet extensive customization

General features

- Support for more than 170 currencies and their symbolic equivalents, getting rates from fixer.io and openexchangerates.org

- Intelligent currency detection, catching everything for most if not all symbols, $, £ and € to EUR, USD, GBP to 'US $'

- Non-intrusive in-site replacement, will never break any website functionality

- Control exactly how your prices are displayed, anything from $100 or 100 dollars to 100 schmekels

- Intelligent localization of multi-currency symbols like $ and kr

- Optional highlighting of prices being converted on pages

- Flip quickly between converted prices and original using either left-clicking, a self chosen shortcut or via the UI

- Mini converter between currencies in the popup window

- Blacklisting and whitelisting of websites for conversion

Changelog

https://github.com/Baizey/UniversalAutomaticCurrencyConverter

This extension/add-on is also available for...

Chrome: https://chrome.google.com/webstore/detail/universal-automatic-curre/hbjagjepkeogombomfeefdmjnclgojli

Firefox: https://addons.mozilla.org/en-US/firefox/addon/ua-currency-converter/