After a power failure, normally Firefox, upon restart, automatically asks if I want to restore my previous tabs. This time, it's not doing that.

This Firefox profile's directory has a sessionstore.js, timestamped just before the power failure, of about 72K, clearly large enough to hold a substantial history. When I start the browser with this profile, all I get is a blank page. If I visit about:sessionrestore, I see the "this is embarrassing" page with an empty list of tabs in it; at the same time, a new sessionstore.js of less than 1K has been put in place, relegating the one with my 72K of data to a .bak file. How can I force Firefox to read my existing sessionstore.js and restore tabs from it?