Safest and quickest is to copy files from the profile folder on the old device to the profile folder on the new device.

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

For the open tabs this would be sessionstore.jsonlz4, only present when Firefox is closed. For the bookmarks you can copy places.sqlite (bookmarks and history) or use a JSON or HTML backup.

• https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them
• https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer
• https://support.mozilla.org/en-US/kb/import-bookmarks-html-file

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

You can copy certain files with Firefox closed from one profile folder to another/current profile folder to transfer your personal data. Note that best is to avoid restoring a full profile folder and only restore important files that are safe to restore/transfer.

• 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) and key4.db (encryption key/primary password) for logins saved in the Password Manager
• cert9.db for 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)

The simplest way is to use Sync. My screenshot shows my other devices and versions of Firefox. I have 1 Mozilla account and they all my bookmarks and logins. You can also send tabs to other devices and see the tabs on the other devices and versions. see screenshot https://support.mozilla.org/en-US/kb/how-do-i-set-sync-my-computer# https://support.mozilla.org/en-US/kb/how-do-i-choose-what-information-sync-firefox#

You would create the Mozilla account on the old device, if you still have it. Then make sure you have the sync settings checked. see screenshot Then sign into your Mozilla account on the new computer and watch the Sync magic take place.

Note that for syncing tabs there is a maximum payload and since you mention 'many open tabs', you can possibly exceed this maximum, especially in case of tabs with a long URL and title.