multiple FF profiles on one computer?

Once again, Christer, you were right: I must have messed up the first (couple of!) time(s) around. I don't know just where I went wrong before, but after uninstalling TB and going through your steps again, my old email and address book are showing up perfectly in the most recent installation of the program.
The only thing I did a little differently from your instructions was when you said, "In the "˜Choose User Profile’ dialogue box highlight "˜Name-1’ > check the box "˜Don’t ask .. "˜ > click Start Firefox. Follow any prompts to configure the email client ", I just X'd out of that configuration screen. In one of my earlier tries, I'd gone ahead and followed through with that, but I think it messed things up for me when I subsequently went to paste the contents of my old profile folder into my D:\Rebecca\Mozilla\Thunderbird folder. I could be wrong about that, maybe it would've worked just the same, but I was using a different server on the old computer, so the configuration info was different, and I didn't want to knowingly load conflicting information.
FYI, I ended up renaming my folder on D: from D:\Rebecca\Mozilla\Thunderbird to D:\Rebecca\Thunderbird profile. While Firefox is installed in a folder called Mozilla, Thunderbird isn't (even though it is a Mozilla product), and I wanted to stick to the original path as closely as possible. Also, it felt clearer to me calling the Thunderbird folder on D: "Thunderbird profile" instead.
OK, now I'm off to try your FF instructions again!

 

Are you sure that TB defaults to the profile on D:?​

It must be defaulting there, because when I click on the TB shortcut on my desktop, TB opens up with all the info from the old computer, and the only place I moved my old TB profile to was D:
But if there's another way of checking, tell me how to do it and I'll verify that.

Now on to Firefox...
FF is working fine when I'm logged in as Gary. I followed your instructions and created a new profile for him, but he's starting with a clean slate here, so I'm not pasting anything from the old computer into his profile.
As for myself (gulp!)... I followed your instructions, and did for my FF profile exactly what I did with TB. FF opens up okay, and the screen is configured the way I had it on the old computer (spoofstick and my google box located above the navigation toolbar), so offhand it looks like things transferred over, but I can't type anything into the navigation bar. When I click on Bookmarks, there's no drop-down screen at all. I think bookmarks is really the only file I need to carry over from the old computer to the new one, so maybe I should just copy and paste that particular component of my profile on the new computer...

 

I wish I knew what I was doing differently each time I try this, because it seems to me as if I'm repeating the exact same steps each time, yet sometimes things work, sometimes they don't!
Firefox is working fine for me logged in as Rebecca now, and my bookmarks have all been transferred. In my early days with FF, I had saved a document with information provided by Ramona (can't remember if it was a posting of hers, or if she'd directed me to a Mozillazine site). This is what was said:

Following is a list of the files to copy to the new Profile:
• bookmarks.html
• cert7/8.db (Security Certificates)
• cookies.txt
• cookperm.txt (cookie and image permissions)
• custom.dic (Dictionary)
• history.mab (Collected Addresses)
• hostperm.1 (Supersedes cookperm.txt in older versions)
• key3.db (Key Database - Certificates, Passwords, etc.)
• persdict.dat (Mozilla Dictionary)
• search.rdf (Search Plugins in the Sidebar)
• user.js (If you have created the file, if not disregard.)
• usercontent.css (If you have created the file, if not disregard.)
• userchrome.css (If you have created the file, if not disregard.)

I had 6 of the above files in my old profile, and I copied them and pasted them into D:\Rebecca\Firefox profile. [cookies.txt and search.rdf were the only two that were already in existence in my new profile (I moved them to a temp folder, as you'd suggested on another occasion, Christer)]
Now everything is hunky dory.
The weird thing is that I'd tried copying/pasting the bookmarks file (478kb) three times previously, but after rebooting, the file was always back to 1kb in size. ??? Hmmm, which reminds me, I don't think I've rebooted since the last editing session of D:\Rebecca\Firefox profile. Guess I'd better try that before I start gloating too much! [I rebooted, and yes, everything's still working!]

THANK YOU CHRISTER!!! (and Westside and Ramona for jumping in too)