Is the history also missing on the old device as that shouldn't happen with using Sync ?

Note that new history is only uploaded for the last 30 days, so you can't use Sync to transfer all history.

You can copy certain files with Firefox closed to the current profile folder to transfer or recover personal data. Note that best is to avoid copying a full profile folder.

You can use the button on the "Help -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page (Root directory).

• Help -> More Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

• bookmarks and history: places.sqlite
• favicons: favicons.sqlite
• bookmark backups: compressed .jsonlz4 JSON backups in the bookmarkbackups folder
• cookies.sqlite for the Cookies
• formhistory.sqlite for saved autocomplete Form Data
• logins.json (encrypted logins;32+) and key4.db (decryption key;58+) for Passwords saved in the Password Manager
  key3.db support ended in 73+; to use key3.db in 58-72, make sure to remove key4.db
• cert9.db (58+) for (intermediate) certificates stored in the Certificate Manager
• persdict.dat for words added to the spell checker dictionary
• permissions.sqlite for Permissions and possibly content-prefs.sqlite for other website specific data (Site Preferences)
• sessionstore.jsonlz4 for open tabs and pinned tabs (see also the sessionstore-backups folder)

Thanks for the reply. Yes, it was missing on the old computer. On the new computer there was only what I did there on that day. And I did know about transferring the places.sqlite file, and I did that yesterday, so now I have the same history file on both (except that since I turned Sync off they differ for yesterday and today). On both computers I'm missing those two years (or a year and a half).

Actually, I still have another laptop, even older, and somehow it picked up the history from after I stopped using it (in 2015)! Up until a year ago when I put it on airplane mode or something (because of the advice I mentioned in my question). So I can still see those missing years, on that computer, but it's not convenient. I have three questions:

1. How did it pick up the history after 2015, if Sync doesn't do more than the last 30 days? (I used it occasionally and left it on a while.)

2. And is there a way I can merge the two history files? The one on the old computer ("Computer 1") and the one on my present computers (Computer 2 and Computer 3)?

3. Is it safe to turn Sync back on?

P.S. Actually, I just checked, and on "Computer 1" some of the history of the last year or two is missing, but before that it seems to be complete, even though I wasn't using that computer. However, it was not turned off -- it was asleep. Could it be that even though it was asleep (with the lid down) it continued to update the Firefox history using Sync? It could be that the holes in the last couple years are when I had it on airplane mode.