The profiles.ini file would need to be edited to add that other Profile, but the format has to be very specific for it to work. Best to not play with editing the profiles.ini file and let Firefox handle the changes needed. Initially it took me multiple attempts to get it right when I first tried to edit the profiles.ini file to add a 2nd Profile.

Use the Profile Manager to create a new Profile, and then copy the contents of the Profile from the flash drive into that new Profile, replacing the existing (new) files.

https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

Thank you! I'd wondered if that might work but hadn't tried it. I've now added it that way. I haven't tried closing firefox and starting with that imported/newly created profile, however.

Is there some way that I can launch it into a new window or something, so I have both the original default profile working in one or more windows, but the newly created profile ALSO running in different windows at the same time? So I can use both without having to close and reopen firefox to switch from one to the other?

I should have noted that I did try going to "about:profiles" and selected the "launch profile in a new browser" button for that newly created/imported profile - it appeared to try to go to the homepage of that profile (drudgereport), but was unable to load it and instead said "Secure Connection Failed" etc.

That profile SHOULD ask me what session I wanted to load (with session manager add on) rather than going to the default homepage - it didn't give me that option, however... maybe the profile doesn't include extensions? I wasn't sure what was going on or why...

Sorry, can't help you with the about:profiles feature, I found it to be an unpredictable piece of garbage; inconsistent in how it works. IMO, that about:profiles feature was added for use by developers on the Aurora and Nightly, and adding it to the Release versions was mistake due to the limited UI which verges upon being nonsensical and counter-intuitive. Overall an unfinished product that needs additional polish.

I have my own system for working with multiple Profiles that I have been using for over 10- years.

To gain a better understanding about how to work with multiple Profiles see this support page: http://forums.mozillazine.org/viewtopic.php?f=23&t=2821799 . Based upon the user running multiple different Firefox installations with their own Profile, but directly compatible with using one Firefox installation with multiple Profiles.

And at the bottom of that page see links to 4 earlier version about Profiles that I did in the years before the one I cited. The two oldest versions have comments added by other users as to how they do it; worth reading to see what others have done.

Thank you The-Edmeister!

Actually I think what you're suggesting may work far better for me anyhow - I really need to use a pre-57 version of firefox with that profile that I had on the flashdrive, because I need some of the extensions that are no longer available on 57+.

If I understand correctly, the ESR versions continue getting security updates far longer than the regular firefox versions? If that's correct, I suppose that's what I ought to use the portable method you're suggesting to install? If so, do you happen to know what ESR version corresponds to the last version of firefox 56?

Also - on my other computer, the one I put a copy of the profile onto a thumbdrive from - I'd started have major problems with it eating up a tremendous amount of memory in a very short time... It's an older laptop with only 4G memory, and in only a few hours or a day maybe, even if I don't have many tabs open, it's eating up as much as 3G memory and then the whole system gets very slow until I close and reopen firefox. I think that happened after I let it update to FF55 or maybe FF56, and I was suspecting that perhaps it had automatically migrated it to a 64 bit version and that could be causing the problem? Does that sound possible? In which case, maybe I ought to switch it to whatever ESR version is comparable to FF54.x rather than FF56.x? Or is there a 56+ 32bit version that I ought to try? Or do you think this is unlikely to be the problem? It just seemed awfully coincidental timing with when I let it update to FF55 I think, or maybe 56.

Any thoughts or suggestions that way?

I wish to heck the new FF was compatible with the old extensions - or that far more extensions had been migrated!

Regardless, thanks for your help!!

Extended Support Release - ESR versions are typically generated every 7 Release versions of Firefox - 39, 45, 52 - but the new ESR version was pushed back to Firefox 60 and is 8 Release versions after the prior ESR.

Yes, security updates are provided for ESR, like every 5 weeks usually coinciding with the full Release schedule - but typically only for 2 additional versions after the new ESR version comes out. With Firefox 60 ESR there will be only one additional security update for ESR 52.0.1; a change from what was done in the past with ESR. https://wiki.mozilla.org/Release_Management/Calendar

Portable method? Keep in mind that mozillaZine thread is already 4 years old, Portable Firefox from Portable Apps has been changed as Firefox has been changed over those years. The advantages that I saw with "Portable" have decreased over time; the biggest obstacle now is that Firefox Portable won't launch when any other firefox.exe process is already running - unless I am missing where the pref to allow for that was moved to. And that 'queers' my use for Firefox Portable for much of anything. Just mentioned that this morning over here - http://forums.mozillazine.org/viewtopic.php?f=7&t=3040180 I have to be able to launch any desktop shortcut for Firefox at will, and not have to close a different version or Profile to get another to launch, so my use of Portable Firefox is almost nil today; and that is an older version (40.0.1) that still lends itself to using that pref - AllowMultipleInstances . I settled on that version of Portable as the "signing" of extensions was supposed to be enforced; didn't bother updating that one as the "enforcement" kept getting pushed back and eventually landed on Firefox 44.

As far as legacy extension compatibility goes, I lost so many extensions long before Quantum arrived on the scene that I stopped updating my main Firefox version long before Firefox 57. For me Firefox 44 was the "deal breaker" for me. I have stuck with ESR 38 since before ESR 45 came out, once I saw what I lost with the full Release 44.0 had done for me. Since then my enthusiasm for Firefox has greatly diminished - Mozilla has managed to ruin (slowly and painfully version-by-version) what was once a great web browser, IMO.