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"You've launched an older version of Firefox" Help! I can't get any of my data...

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I installed a dev version of Firefox to test a plugin.

Then I uninstalled the dev version, but now none of my profiles on the regular version work...

Please help me, I really need my old browser history and cookies.

When I try to use one of my old profiles, I get "You've launched an older version of Firefox" and it tells me to either Create New Profile or Exit.

I read online that people add "-allow-downgrade" to the Windows shortcut, but this does nothing for me.


Thanks you and please help!

I installed a dev version of Firefox to test a plugin. Then I uninstalled the dev version, but now none of my profiles on the regular version work... Please help me, I really need my old browser history and cookies. When I try to use one of my old profiles, I get "You've launched an older version of Firefox" and it tells me to either Create New Profile or Exit. I read online that people add "-allow-downgrade" to the Windows shortcut, but this does nothing for me. Thanks you and please help!

All Replies (1)

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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?

Modified by jscher2000 - Support Volunteer

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