I would suggest using sync.

Type about:accounts<enter> in the address bar. [57+] https://accounts.firefox.com/settings

https://support.mozilla.org/en-US/kb/how-do-i-set-sync-my-computer Sync your bookmarks, history, passwords, add-ons and open tabs with another copy of Firefox. Learn how to set up Sync.

https://support.mozilla.org/en-US/kb/how-do-i-choose-what-types-information-sync-firefox We'll show you how to choose which types of information (bookmarks, history, add-ons and passwords) to share across all your devices through Firefox Sync.

https://support.mozilla.org/en-US/kb/firefox-sync-troubleshooting-and-tips

https://support.mozilla.org/en-US/kb/sync-firefox-bookmarks-and-browsing-history-iOS Sync your passwords, history, tabs and other browsing information across your iOS, Android and desktop computers with Firefox Accounts.

I don't know whether Sync can do it; interesting idea.

Otherwise, you might use add-ons or external tools to extract the history to a separate format such as Excel, CSV, etc., and merge the data externally.

Example of an external program which can read Places.sqlite database files: http://www.nirsoft.net/utils/mozilla_history_view.html

Thank you for the idea. This utility allows me to read history. But how do I merge the files so I only have one places.sqlite for use directly in firefox?

I don't know whether there is a way to insert new history entries from outside of Firefox.