This isn't an entirely satisfactory solution, since I would prefer to open Firefox and simply have the old tabs open like they used to do, but you can restore your previous session each time you open Firefox: (Menu Bar) History > Restore Previous Session. Worked like a charm for me.

If you always open the last set of tabs, an alternative approach is this:

1. Click the orange Firefox button, then select options to open the options window
2. Go to the General panel
3. Change the setting "When Firefox starts" to "Show my windows and tabs from last time"

Firefox 4 automatically saves the session, if it is not saving the session, the file that stores the details may be corrupt. To fix that first open your profile folder, for details of how to find it see https://support.mozilla.com/kb/Profiles

Now close Firefox and then delete sessionstore.js in the profile folder. A new version of this file will be created when you next start Firefox.

I'm having this issue too, except I've had Firefox 4 for a while now. This just occurred in past few days. I normally close Firefox and then reopen it in the morning, but when I did all of my tabs were gone. I didn't change a thing. It opened with just one tab, which is my homepage, and it didn't even have me signed in like it's supposed to.