hello, this might happen when firefox is not able to properly save preferences in its profile folder. please try this: Click the menu button ≡ and then click help ? > troubleshooting information > profile folder - "show folder". then a new window will open up. in this window look out for a file named user.js (it might be used to overwrite your custom settings). in case it is present , delete or rename this file and restart firefox afterwards.

for more information and other steps please also see: How to fix preferences that won't save

Thanks for the quick reply. After posting the question, and before you answer, I got in a destructive mood and deleted user.js and Pref.js. This seemed to cure the problem. On hint for others. Before deleting these files, rub FF and create a bookmark folder with the "home" pages (those settings will be lost with the files). Shut down Ff and delete the files. Then re-start. Use the bookmark folder to start all the original "Home" pages then set them on the options again. The only other puzzle I have is, why are all my different profiles affected in the same way after the update?