The icons for the search engines are outdated, and lacks the additional functionality that http://addons.mozilla.org/firefox/addon/search_by_image provides.

Have used this extension for a very long time. Works like a charm

Edit: Moved to "Search by Image" add-on by Armin Sebastian. This one still works, but alternative is actively developed, supports more engines out of the box, detects images behind elements that this add-on cannot detect and more.

Can't test this extension. Does nothing on Firefox on Android

It requests too much permissions (https://addons.mozilla.org/cs/firefox/addon/fast-image-research/ requests less). Can NOT find link to open source platform like a Github.