Your Firefox Developer Edition version was the beta of Firefox 128 and your regular version is Firefox 127? While there is some risk of incompatibility of files as a result of being used in the later version, it's worth a try to use them in Firefox 127 again since there was only a one version difference. The -allow-downgrade switch should work ([[Dedicated profiles per Firefox installation]]), but if that isn't working, there is a lesser known option. This requires modifying a file in the profile folder you want to use. So here's how: (1) Start Firefox and create a new profile if needed (2) Type or paste '''about:profiles''' in the address bar and press Enter/Return to load the About Profiles page. This page should list at least two profiles. The profile that Firefox is currently using will have this: '''This is the profile in use and it cannot be deleted.''' (3) For the other profiles you want to use in Firefox 127: * Click the Open Folder button on the Root Directory row * Find the '''compatibility.ini''' file (by default, Windows may hide the .ini extension, but there's only one file whose name starts with compatibility) * Right-click the '''compatibility.ini''' file and choose Rename, then insert OLD so the name reads '''compatibilityOLD.ini''' (4) Back on the about:profiles page, click the '''Launch profile in new browser''' button for the modified profile. Does it run in Firefox 127? (5) Assuming using the upgraded files doesn't cause new problems, for your real profile, click the '''Set as default profile''' button for that profile so Firefox uses it automatically at the next startup. Did that work?