When you 'set up a fresh operating system,' do you mean you did a system refresh? If so, did you select to save your data?

Using the computer's file browser, open your user folder. Look for a folder called Windows.old

You need to place a sessionstore.jsonlz4 in the current profile folder to restore that last session.

Location used for the main profile that keeps your personal data (Root Directory on about:profiles).

• C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\Profiles\<profile>
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

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.

• 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)

When you 'set up a fresh operating system,' do you mean you did a system refresh? If so, did you select to save your data? Using the computer's file browser, open your user folder. Look for a folder called Windows.old

I was running linux before and needed windows so I deleted all my old data and installed windows. I figured out how to open all my old tabs.. I clicked the 3 lines in firefox, clicked my username and scrolling down could see my old tabs underneath the signout option. I was able to click each one individually to open the old tabs. Thank you so much for your help!

That was very good work. Well Done. Please flag your last post as Solved Problem as this can help others with similar problems. Go to that post and click the 'Solved' button to its right.