How to remove `t=canonical` from Firefox browser searches made on DuckDuckGo?

&t=

Through partnerships with developers and companies, DuckDuckGo has been integrated into many applications. In these partnerships, a portion of DuckDuckGo's advertising revenue is sometimes shared back. To assign advertising revenue and collect anonymous aggregate usage information, developers add a unique "&t=" parameter to searches made through their applications.

If you want to remove it, edit the file:

/usr/lib/firefox/distribution/searchplugins/locale/*language-here*

Edit all the languages just to be sure.

Edit for 20.04

1. Open Firefox, navigate to Preferences --> Search, and remove the per-installed DuckDuckGo search engine
2. Download the search engine new from the Firefox Add-ons directory. Select this as the default search engine in the preferences The string, &t=canonícal, ceased to insert itself into the searches via the address bar thereafter. With help from Komalleana Byzantis

Below are before and after pictures:

^{Source:DuckDuckGo Forums}

Read the output from apt-cache show xul-ext-ubufox. Here's a partial quote:

Package: xul-ext-ubufox
Priority: optional
Section: web
Installed-Size: 378
Maintainer: Ubuntu Mozilla Team <[email protected]>
Architecture: all
Source: ubufox
Version: 2.8-0ubuntu1
Replaces: ubufox (<< 0.9~rc2-0ubuntu3)
Provides: firefox-ubufox, ubufox
Depends: aptdaemon, libglib2.0-0 (>= 2.26)
Recommends: firefox (>= 9.0)
...
Description-en: Ubuntu-specific configuration defaults and apt support for Firefox
 Adds Ubuntu-specific modifications to Firefox.
 .
 Integrates the browser with Ubuntu to:
  * Enable searching for missing plugins from Ubuntu software catalog
  * Add the following options to the Help menu
    - Get help on-line
    - Help translating Firefox
    - Ubuntu Release Notes
  * Set homepage to Ubuntu Start Page
  * Display a restart notification after upgrading Firefox
  * Add ask.com to the search engines.
 .
You can uninstall this if you prefer to use a pristine Firefox install.
Enhances: firefox
Homepage: https://launchpad.net/ubufox

Note the third-last line.

As for why, Canonical may earn some revenue but that's just speculation.

I am using Firefox 60.0.1 on Xubuntu 16.04.4 (64-bit). I had the same problem you face. The chosen answer provided by Mitch did not work for me. Instead, I resolved the issue by following this procedure:

1. Open Firefox, navigate to Preferences -> Search, and remove the pre-installed DuckDuckGo search engine
2. Download the search engine anew from the Firefox Add-ons directory; it can be found here
3. Select this as the default search engine in the preferences

The string, &t=canonical, ceased to insert itself into my searches via the address bar thereafter.

I cannot tell you why the behaviour is there in the first place.