Since Firefox has an issue on sending data via POST forms from WebExtensions (see here), we have to post the data through a page that is formed as a so-called "top-level data URI" page. Such a technics got misused and therefore disabled in web browsers.

Since we are restricted here (until the bug linked above gets fixed), either top-level data URIs have to be enabled, or we can not send the image data to Bing (notice that we can deal with the other search providers though, and with URLs or at Chrome too). If this browser is not Firefox, filefile a notice on it, please.

The current state of the option can be checked at the browser config "about:config" page, under the "security.data_uri.block_toplevel_data_uri_navigations" key.