You can't. If Firefox 20 is in Private Browsing mode then a possible persona is disabled and the default theme is used. So you will have to make a choice.

I don't understand something. I'm also on Firefox 20 and on the Options menu I have everything set to clear all history and cookies when Firefox closes. Both "Remember Browsing and Download History" and "Remember Search and Form History" are unchecked (i.e., not to be remembered). I have no issues with personas nor am I seeing any option for private browsing tabs. I am fanatically opposed to histories and recents, and with these settings I've never had a problem. Don't my settings come under the term "private browsing"?

I may add that as an added measure, I religiously use Ccleaner and Bleachbit to wipe away any residues that may have been left behind.

Yes, those items are a part of the Private Browsing "mode".

https://support.mozilla.org/en-US/kb/private-browsing-browse-web-without-saving-info

If by this definition I have private browsing on, then why do my personas work and why am I not seeing that bizarre purple bar with the blindfold? I'm set to Custom History Settings and both "Remember Browsing and Download History" and "Remember Search and Form History" are unchecked. I hate any software or OS that keeps meticulous records of everything I do.

Seriously? Are you trying to purposely alienate some of your users?

How bout you tell us how to get back to Firefox 19 and then disable auto-update, that would help tremendously since I shouldn't have to "make a choice."

What exactly do you mean by "purposely alienate some of your users"? What users? Sadly the surfing the net today has become a total war against ads, tracking, cookies, analytics and other intrusive activity. I also don't like the idea of any software keeping meticulous histories and logs of everything I do. What I did yesterday is not what I'll be going today, and I prefer others not know what I've done or where I've been.

I'm under the impression that, "private browsing" or otherwise, the settings I use still do a pretty good job at hiding all this, with a little help from some add-ons. I also have 3 cleaners I use when I'm done, just to be safe.