The URLs are kept in your Firefox profile folder, in a Json file called sessionstore.js. The same folder also contains sessionstore.bak, which is automatically generated by Firefox every 15 seconds. I have just learned all this stuff because I have had the same problem this week. Before you do ANYTHING make copies of these two files in another place! After that is done you can try closing Firefox, removing the existing sessionstore.js file and renaming sessionstore.bak to sessionstore.js. then try to re-open Firefox. With luck, Firefox will then open normally. If both files are corrupt there are ways to find older versions of the sessionstore.js file, but i don't feel qualified to go into that as I am still working on my own problem at this stage. All the best, KK.