I would recommend switching the multistage code to use the `SHOW_FIREFOX_ACCOUNTS` special message action instead of `OPEN_URL`, since that already dynamically gets the right Firefox accounts endpoint. (see https://firefox-source-docs.mozilla.org/toolkit/components/messaging-system/docs/SpecialMessageActionSchemas/index.html?highlight=show_firefox_accounts#show-firefox-accounts) You will need to extend the SMA to accept more URL parameters though (like all your flow params) and encode them in the querystring.
Bug 1657976 Comment 1 Edit History
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I would recommend switching the multistage code to use the `SHOW_FIREFOX_ACCOUNTS` special message action instead of `OPEN_URL`, since that already dynamically gets the right Firefox accounts endpoint. (see https://firefox-source-docs.mozilla.org/toolkit/components/messaging-system/docs/SpecialMessageActionSchemas/index.html?highlight=show_firefox_accounts#show-firefox-accounts) You will need to extend the SMA to accept `extraParams` (like all your flow params).