mikael.boldt, I wouldn't delete any profiles as one of them may contain the missing bookmarks and passwords etc.

To try and see if one of them does, follow this guide: https://support.mozilla.org/en-US/kb/recover-user-data-missing-after-firefox-update

If you can't launch Firefox because of the 'used with a newer version' message then you can either create a new profile as suggested, or you can bypass Firefox's downgrade protection and force it to open with a profile from a newer version with the "-allow-downgrade" command line argument.

See here for more information on using Firefox with a command line argument, but note the caution under the "-allow-downgrade" heading: https://wiki.mozilla.org/Firefox/CommandLineOptions

OK. Now I have collected all my usernames and password. However, I still get this error: "This profile was last used with a newer version of this application. Please create a new profile."

I don't know what you are doing or why you are getting the "Please create a new profile" error but, to force Firefox to start with a profile used in a newer version:

Right-click the Windows Start button, select Run, enter one of the following commands in the Run dialog and click OK:

firefox.exe -P -allow-downgrade

.... or, use the full path to firefox.exe enclosed in quotes, for example:

"C:\Program Files\Mozilla Firefox\firefox.exe" -P -allow-downgrade

The Firefox - Choose User Profile window should open.

Select the profile you wish to use in the Firefox - Choose User Profile window and click Start Firefox.