My impression is that you're using a fork of Firefox for Android that is customized to the point of breaking essential web APIs, but you'd have to share more details in a GitHub issue to figure out what went wrong on your device.

https://github.com/dessant/search-by-image/issues

The extension allows you to configure the toolbar button as a shortcut for quickly searching with a single search engine or a group of search engines. The toolbar button opens a popup by default, where you can select a search engine, and there is also a button for opening the extension's options.

Though that's not the standard way of accessing a browser extension's options, it was just added to the popup as a convenience. All browsers have buttons for opening options on the management pages of extensions. Visit about:addons to open the options page of your extensions.

Google Lens is already supported, you can enable the search engine from the extension's options.

The extension uploads the image you select to the image search service, and does not add keywords to searches.

What you're experiencing is Google altering the search results page for reverse image searches, which is outside the control of the extension.

The extension cannot add keywords to image searches, that's probably done by the search engine by classifying the image. Open an issue on GitHub if you need help, and please start by mentioning which search engine you're using.

https://github.com/dessant/search-by-image/issues

There is a search mode to select a portion of the image, please read the description.