You can possibly export the logins to a CSV file in each profile and import the CSV files.

• https://support.mozilla.org/en-US/kb/export-login-data-firefox-lockwise

See the note at the bottom of this article about how to enable CSV import via the about:config page.

• https://support.mozilla.org/en-US/kb/import-login-data-file
• about:config => signon.management.page.fileImport.enabled = true

You can open the about:config page via the location/address bar. You can click the button to "Accept the Risk and Continue".

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

People (not just here, I read a bunch of support questions) uses the word "import" without specifying whether it overwrites existing entries, discards conflicting entries, or offers the user a choice, let alone what tell information is available to assist in the choice.

Lacking that information, exporting BOTH lists and merging manually seems to be the only risk-free approach.

We use import to indicate that it merges the logins like in case of importing a CSV for logins or an HTML backup for bookmarks. We use replace in case restoring a backup replaces the current data like with restoring a JSON backup for bookmarks that replaces the current bookmarks.

"We use import to indicate that it merges the logins like in case of importing a CSV for logins"

Yes, but how do THEY do it? Do I need to go experiment with one of them? Is the process documented somewhere? If so, where?

Which data overrides which data? Do I get to choose which data to keep? Can I choose to keep both, or edit one or the other? Does the process simply generate duplicate entries? Do I need to manually massage the data before merging, like one does when importing email logins?

This would ideally be explained the first time the password manager documentation uses the word "import".