Canonical — Web Guy

Allows you to quickly navigate to or copy the canonical version of the URL you're on, if available.

What?
In a nutshell, a canonical link is the "clean" or official version of a webpage URL.

Example of a non-canonical URL:
https://www.amazon.com/Apple-iPhone-GSM-Unlocked-5-8/dp/B075QMZH2L/ref=sr_1_3?s=wireless&ie=UTF8&qid=1516789917&sr=1-3&keywords=iphone+x

Example of a canonical URL:
https://www.amazon.com/Apple-iPhone-GSM-Unlocked-5-8/dp/B075QMZH2L

See the difference? One has tracking info, tags, and queries, which are generally only useful for the site owner, where the cleaner, more human-readable version is better for use with: bookmarking, linking to from a website or social media, marketing materials, and search engines.

Canonical links are generally thought of as only being useful for webmasters who would like to avoid SEO issues such as duplicate content in search engines like Google. However, they have many additional benefits for organization and productivity not often considered.

Why?

• Professionalism — If you write online: blog articles, help forums, product documentation, etc., you should be using the canonical version of URLs to share with people. Anything else is sloppy and a lack of attention to detail.
• Attribution — Some websites re-post content from other websites. The correct way to do this without plagiarizing, is to credit the official page/author with a canonical link. This helps you to avoid accidentally crediting the wrong source.
• Bookmarking — If you make sure to bookmark the official URL in the first place, there's a better chance that it won't become broken later.
• Social Media — Presentation and saving characters for your message (especially on Twitter).
• Privacy — It's very rare, but some URLs contain private or personally-identifiable information meant only for you, which you wouldn't want to copy and paste into a public space.

This list is not a master list of all possible uses for this add-on and the canonical link in general, and I would be interested to hear from users if they've discovered other uses for it.

How?

1. If you see no "C" icon to the right of the URL at the top of your browser window, then the website is not declaring a canonical link.
2. If you see a blue "C" icon, it means that a canonical version is available, but you're not on it. Click the icon to redirect to the canonical version or right-click to copy it without going to it.
3. If you see a gray "C" icon, it means that a canonical link is declared and you're already on it.

Credits
This add-on is inspired by the Canonical feature of the now retired SearchStatus add-on, is designed by Web Guy, and is coded with major assistance from Jeremy Schomery. Thanks to @Enyby and @Cimbali for additional improvements.