Firefox add-on is shown beside search box on desktop, but where is it on Android? I installed Search by Image Firefox add-on. But I can't see the extension button that we see next to search box on desktop version.
Where do I find it?
Firefox add-ons for Android by default do not appear next to the search box, unless the add-on has an option to show it there.

You need to tap the 3 dot (⋮) menu to reveal them. They could be separately listed as with this example or shown under the add-ons (blue circle with arrow) 

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