Firefox 4 saves the previous session automatically, so there is no longer need for the dialog asking if you want to save the current session.
You can use "Firefox > History > Restore Previous Session" to get the previous session.
There is also a "Restore Previous Session" button on the default about:home Home page.

Another possibility is to use:

• Tools > Options > General > Startup: "When Firefox Starts": "Show my windows and tabs from last time"

I have the same problem, and i have my firefox set to " show my windows and tabs from last time " set on as defaults. but each time i open up my firefox i get the tabs from the time when i updated from 3,xxxx to 4:-(((((

"Firefox > History > Restore Previous Session" is not porsible to sellect when i open firefox up. Im running Win7 64 Ultimate.

If you get tabs from an older session then it is possible that there is a problem with the files sessionstore.js and sessionstore.bak in the Profile Folder

Delete sessionstore.js and sessionstore.bak in the Profile Folder

• Help > Troubleshooting Information > Profile Directory: Open Containing Folder

If you see files sessionstore-##.js with a number in the left part of the name like sessionstore-1.js then delete those as well.

Deleting sessionstore.js will cause App Tabs and Tab Groups to get lost, so you will have to create them again (make a note).

See:

• http://kb.mozillazine.org/Session_Restore