Hi

Do you have session restore enabled? This will bring back tabs that you had opened when you next start Firefox.

This support article should help:

https://support.mozilla.org/en-US/kb/restore-previous-session#w_configuring-session-restore

In case you use "Clear history when Firefox closes".

• do not clear the "Browsing history"

• Settings -> Privacy & Security
  Firefox will: "Use custom settings for history":
  [X] "Clear history when Firefox closes" -> Settings
• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

• using "Clear history when Firefox closes" to clear the "Browsing history" includes the pinned tabs as those tabs are considered history
• using "Clear history when Firefox closes" in Firefox 102+ honors exceptions and keeps cookies with an allow exception, previous versions removed all cookies
• clearing "Site settings" clears exceptions for cookies, images, pop-up windows, and software installation and exceptions for passwords and other website specific data