Hi, not sure what happened there.

Please see if you can restore your data as shown in -

• Profiles - Where Firefox stores your bookmarks, passwords and other user data.

• Recover important data from an old profile.

If your question is resolved by this or another answer, please take a minute to let us know. Thank you!

Is it just a matter, then, of replacing bit and pieces — bookmarks, passwords, extensions, etc. — from my old setup (Firefox 48.0) into the new default version of Firefox (48.0.1)? As you can see in the screen shot, my old profile is still there in the trash. Is it possible to just replace the old profile into where the new, default profile lies? There must be a relatively painless, effective way to do this; apparently other people are having the same problem I am with this new update. Thanks for further advice.

You should be able to replace the entire profile - please read this article carefully: Back up and restore information in Firefox profiles.

Also: Profile Manager - Create, remove or switch Firefox profiles

Thanks for the suggestion.

I went ahead and tried to restore the old profiles folder (sitting in trash) by dragging the folder itself over the new Firefox default profiles folder (they have exactly the same name), and was prompted if I wanted to overwrite and replace it. I indicated yes. When I reopened Firefox, however, there was no change. Version 48.0.1 still opens in brand-new default mode, while the old version (48.0) remains in the trash, where it was automatically dumped during an update. Am I doing everything correctly, or is there anything else I should be doing?

Since it happened when you updated FF, it's possible that an extension (add-on/theme) is no longer compatible and is somehow corrupting the profile. It might be a good idea to try copying other (safe) data such as bookmarks etc., one at a time first to see if that works, and leave extensions till last.

I went ahead and tried that, and in the process some data apparently got altered and my old (trashed) version of Firefox 48.0 could no longer open. So, I lost everything. I went ahead and started using Firefox 48.0.1 from scratch, and am now in the process of re-adding everything that I can remember (bookmarks, extensions, etc.). Thanks anyway for all the advice; it was very much appreciated.

But this issue of Firefox dumping the old version during a new update is bound to come up again — for me and for for others — and I will be following it up and sharing anything I can find with folks here in the future.