Hello! This can happen because your profile is still on an old version of Firefox. To fix this, follow these steps:

1. Type about:profiles in the search bar and press enter.

2. Find your profile, and click Open folder.

3. Your profile folders will open. Find a file named Compatibility.ini.

4. Open it, and where it says "LastVersion=139.0.4_20250517143237/20250517143237", change the version, so that 139.0.4 , changes to 140.0.4. I advise you to backup your data and profile!

I hope you manage to solve the problem!

Vexi

Thanks for the reply. I do not have the option OPEN FOLDER in about:profiles. I assume you just add the latter to the web brower not a search within the firefox account?

Lastly - How do you back up profile? Should I do this once restored?

Hello! You need to open the root folder of the profile you are using. If you don't have the Open folder option, go to Explorer and in the search bar type C\Users\your name\AppData\Roaming\Mozilla\Firefox\Profiles and open the folder of the profile you want. When you open it, you will find the Compatibility.ini file there. I am also sending you a screenshot of where Compatibility.ini is in the attachment.

To back up a profile, just copy and paste the folder of the profile. If you have an account created, you don't need to do this!

I hope this helped you!

Vexi

The compatibility.ini file was already set to 140.0.4. I did notice in my profile is says prfile:default - could this be the problem? My iPhone still has all the data I'm looking for (bookmarks and autofills).

Hello! profile:default is the default profile. If you have multiple profiles, try selecting a different one. You can also try logging out of your browser and logging in again. You can also create another profile and log in with your account.

Thanks again for the reply. Profile:default is good to know that's as it should be. If this isn't the compatibility file what else could it be?

I don't know yet. Wait until the next update. Try installing Firefox Beta, because it's a newer version. Try signing up for Beta. You can install it separately from Firefox.

OK, so all data is back. I deleted firefox and re-insalled it. Many thanks

Great that you succeeded!