Reset the browser.startup.homepage_override.mstone pref via the right-click context menu to the default on the about:config page and close and restart Firefox to restore the about:home page to its default.

• Open the about:config page via the location bar
• Type in the Filter bar: mstone
• Right-click the browser.startup.homepage_override.mstone line and select: Reset
• Close and restart Firefox

If that didn't help then delete the delete the chromeappsstore.sqlite file in the Firefox Profile Folder and repeat the above.

You can use this button to go to the Firefox profile folder:

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

I am going to need more detailed instructions on how to perform the instructions given in your solution. I am not an "under-the-hood" computer geek, just an old guy computer user.....ray.martin3@gmail.com

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.

• Use the Filter bar at the top of the about:config page to locate a preference more easily.

Filter: mstone

You can double click the line with the browser.startup.homepage_override.mstone pref and clear that field and click OK to save the empty string value.
Close and restart Firefox to see if that has helped.

See also:

• http://kb.mozillazine.org/about:config

Followed your instructions and got to the Enter String Value Window. The phraserv9.0.1 appeared in the window. I deleted it and clicked on OK, closed and restarted Firefox. No luck. Followed same procedure, but left rv9.0.1 in-place, closed and restarted. Same result. What if I remove Firefox from the program files folder using the add/remove tool, then download and reinstall Firefox?

Try to the delete the chromeappsstore.sqlite file in the Firefox Profile Folder and repeat the above.

You can use this button to go to the Firefox profile folder:

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

Create a new profile as a test to check if your current profile is causing the problems.

See "Basic Troubleshooting: Make a new profile":

• https://support.mozilla.org/kb/Basic+Troubleshooting#w_8-make-a-new-profile

If that new profile works then you can transfer some files from the old profile to that new profile, but be careful not to copy corrupted files.

See:

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

Removing chromeappsstore.sqlite did not help, but creating a new profile worked. Now, how do I find the bug in the old profile before transing files to the new profile?

Try to only move files that you really need like bookmarks (places.sqlite or restore bookmarks from a JSON backup in the bookmarkbackups folder) and saved names and passwords (key3.db and signons.sqlite) and other saved form data (formhistory.sqlite) and make a backup of files before replacing them, so you can revert if the problem returns.

• http://kb.mozillazine.org/Backing_up_and_restoring_bookmarks_-_Firefox

Bookmark Export/Import done. Everything is working fine now. Thanks.