Have a look here. https://support.mozilla.org/en-US/questions/1442499#answer-1642570

See also:

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

Does Firefox create and update a recovery files in the sessionstore-backups folder when Firefox is running?

• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

You will normally find these files in the sessionstore-backups folder:

• previous.jsonlz4 (cleanBackup: copy of sessionstore.jsonlz4 from previous session that was loaded successfully)
• recovery.jsonlz4 (latest version of sessionstore.jsonlz4 written during runtime)
• recovery.baklz4 (previous version of sessionstore.jsonlz4 written during runtime)
• upgrade.jsonlz4-<build_id> (backup created during an upgrade of Firefox)

Make sure Firefox isn't set to run in permanent Private Browsing mode (Always use Private Browsing mode; Never Remember History).

• https://support.mozilla.org/en-US/kb/private-browsing-use-firefox-without-history

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" 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

Have a look here. https://support.mozilla.org/en-US/questions/1442499#answer-1642570

I am not concerned now about getting the lost tabs, what's gone is gone, but I need to fix the issue, the sessions are never restored, and it only happens after I login to sync my account, it's a sync issue.

See also:

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

Does Firefox create and update a recovery files in the sessionstore-backups folder when Firefox is running?

• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

You will normally find these files in the sessionstore-backups folder:

• previous.jsonlz4 (cleanBackup: copy of sessionstore.jsonlz4 from previous session that was loaded successfully)
• recovery.jsonlz4 (latest version of sessionstore.jsonlz4 written during runtime)
• recovery.baklz4 (previous version of sessionstore.jsonlz4 written during runtime)
• upgrade.jsonlz4-<build_id> (backup created during an upgrade of Firefox)

Make sure Firefox isn't set to run in permanent Private Browsing mode (Always use Private Browsing mode; Never Remember History).

• https://support.mozilla.org/en-US/kb/private-browsing-use-firefox-without-history

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" 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

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

Did all of this before, no luck, even the menu is greyed out as if there was no tabs at all.

>Make sure Firefox isn't set to run in permanent Private Browsing mode (Always use Private Browsing mode; Never Remember History).

It is not, all settings are checked to restore the session.

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

I keep the history too.

Again, my observation is when I login to the sync, it screw up the profile and it doesn't restore the sessions anymore.. tried with a clean firefox installation on different OS, and even in librewolf, as there's an option to disable the sync, once I have that disabled, deleted the old profile and created a new one, the session restores normally. So what is in the sync or the profile that make it that way?! This only been happening in the past month or so, I'm sure a new update or changes to how sync works caused this.

What items do you sync? Does removing "Open tabs" from Sync work?

• https://support.mozilla.org/en-US/kb/how-do-i-choose-what-types-information-sync-firefox