I have several tab groups which have been working successfully.
Today whenever I open a tab group the number of tabs is set to zero and all my tabs are lost. In the Tab Group List they show as 7, 19 etc.
You can check for problems with the sessionstore.js and sessionstore.bak files in the Firefox Profile Folder that store session data.
Delete the sessionstore.js file and possible sessionstore-##.js files with a number and sessionstore.bak in the Firefox Profile Folder.
Deleting sessionstore.js will cause App Tabs and Tab Groups and open and closed (undo) tabs to get lost, so you will have to create them again (make a note or bookmark them).
Deleting sessionstore.js does not solve the problem. Whenever I open a tab group it is shown as 0 tabs.
I inspected the files in C:\Users\Andrew\AppData\Roaming\Mozilla\Firefox\Profiles\20kbvg1h.default\tabgroupsmanagerdata\save_data and they contain all the expected tab data.
If the TGM extension isn't working properly then you need to contact the developer to see if there is an update that addresses this issue or if it can be solved otherwise.
See also [/questions/956781]
I too have the similar problem...
But, for me, a needed plugin is cause of the problem.
Plugin is: Greasemonkey
Problem is: When the restart the firefox (manually OR automatically), tab groups are not saving the data.
Please find me the solution...