Never saw that Firefox imports Edge bookmark unless the user themselves does this action to import.

You are correct. I initially approved importing, but changed my mind. Despite my uninstalling and clearing cookies, Firefox remembers my choice and imports automatically. Is there a way to fix this by either preventing this at installation or by managing the imported bookmarks after installation?

Firefox would only import Edge bookmarks if you choose this in the import wizard that Firefox may show if you select "File -> Import from Another Browser" or use the Library. This should never happen without your consent.

• https://support.mozilla.org/en-US/kb/import-bookmarks-internet-explorer-or-microsoft-edge