Yes, but I don't completely trust Sync, so I recommend backing up your critical files. Please see the following article:

Back up and restore information in Firefox profiles

In particular, make sure you have:

• sessionstore.jsonlz4 file (created by Firefox when it closes)
• entire sessionstore-backups folder (live session tabs and some older backups)

As for bookmarks, saved logins, etc., the following article mentions the key files:

Recover important data from an old profile

Yes, but I don't completely trust Sync, so I recommend backing up your critical files. Please see the following article: Back up and restore information in Firefox profiles In particular, make sure you have:

• sessionstore.jsonlz4 file (created by Firefox when it closes)
• entire sessionstore-backups folder (live session tabs and some older backups)

As for bookmarks, saved logins, etc., the following article mentions the key files: Recover important data from an old profile

So basically these sessionstore files are keeping record of my live opened tabs and when I open them on a new OS install they will load my opened tabs? To see if I understand it correctly.

At startup, Firefox checks for previous session files. There isn't an internal function of Firefox to use anything except the last session it finds in your currently active profile folder.

You can substitute the older files for the newer (useless) ones to roll back a new installation to a previous set of tabs. This requires manual file swaps while Firefox is closed, since Firefox itself doesn't have a feature to load alternate files after startup.

This is an old write-up from Windows 10.

To try swapping in an older session history file so Firefox will load it, the standard "manual swap" method is as follows (assuming Firefox is still running):

(1) From inside Firefox, open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" Help > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, click the "Open Folder" button. This should launch a Windows File Explorer window listing the various files and folders in your profile.

(2) Leaving that window open, switch back to Firefox and Exit, either:

• "3-bar" menu button > "power" button Exit
• (menu bar) File > Exit

Pause while Firefox finishes its cleanup -- a file named sessionstore.jsonlz4 should appear in the folder.

(3) Once Firefox has stopped updating things, while viewing the main level of the profile folder, right-click > Rename sessionstore.jsonlz4 to sessionstoreOLD.jsonlz4

If you see an older sessionstore.js or sessionstore.bak file, move it somewhere else or delete it.

(4) Copy in the other file you want to restore and, if needed, rename it with the name appropriate for its format:

• Compressed file such as recovery.jsonlz4 => sessionstore.jsonlz4
• Uncompressed file such as recovery.js => sessionstore.js

(5) Start Firefox and it should read the swapped in sessionstore file and restore whatever was in the substitute file; if you do not have Firefox set to restore your previous windows and tabs automatically, use History > Restore Previous Session