You will require access to the primary email address to be able to use Sync as for connecting another device by using the email address and password you may also have to supply a code send to your primary email address. A recovery key only helps if you forgot the password, you can't use it if you lose access to the email address.

• https://support.mozilla.org/en-US/kb/ive-lost-my-firefox-sync-account-information
• https://support.mozilla.org/en-US/kb/change-primary-email-address-firefox-accounts
• https://support.mozilla.org/en-US/kb/add-secondary-email-address-firefox-accounts

You can always use a USB drive to transfer your personal data directly in case of problems with sync.

• https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile

For your tabs you need sessionstore.jsonlz4.

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.

• 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. I have backed up the bookmarks then imported them into FF on Linux. I think I will still need to change my primary email address on the desktop (Win10). Is the best way to do that: delete the primary email address and make the secondary email address the primary address? Is that possible?

Your past Primary email is needed to do the verification to change the secondary or any other email to the Primary for this action to happen. So if you loose access to Primary before doing this change there nothing that can be done to change this if you loose access to Primary before you make the change.