https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer and https://support.mozilla.org/en-US/kb/import-bookmarks-html-file and https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles and https://support.mozilla.org/en-US/kb/recover-lost-or-missing-bookmarks and https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Please let us know if this solved your issue or if need further assistance.

fwustner said

I'm tempted to just drag-and-drop everything in there into the new profile buried in the User/AppData directory, but I'm a bit worried about whether that could have any unintended consequences. Does anyone here have a recommendation?

I have done that frequently - but you need to use the Windows contextual menu (right-click) command to Copy & Replace existing files that have the same name.

As to how good that works for you depends upon how old the files in that Old Firefox Data are and what version of Firefox they came from.

Firefox 55 carried a number or new & changed files, and we have seen varied results when a user goes back from Firefox 55 - 57 to Firefox 54, 53, or 52 versions.

Thank you both for the help and advice.

To answer the issue you brought up, edmeister, I was taking settings from (I think) version 56 and trying to restore them in version 57. The details are unimportant, my pc just had a sudden unrelated crisis right when the update to 57 came out, and I had to do some cleaning and restoring.

Again, thanks for the assistance.