I am going to answer this myself.

I opened up omni.ja and did a lot of grepping and reading code. While I was reading code, I saw several references to pref("mail.folder_widget.max_recent") and totally missed what it meant. Then I finally got it--Mozilla finally put it in the config settings!!

To change the value of max recent folders, I went into Options | General | Config Editor and entered max_recent in the search. Voila! Done!

Enjoy!

I found this, too, however it isn't quite the whole story.

I increased my "recent folders" setting to 35, but I still noticed that I was still getting less folders. It appears that there is also a maximum age for a folder for it to be considered "recent"

after a bit of digging, I found the file

modules\folderUtils.jsm

in the Omni.ja library. (a zip file than you can extract into a folder structure containing the Thunderbird code)

This contains the function getMostRecentFolders >>>>>> function getMostRecentFolders(aFolderList, aMaxHits, aTimeProperty) {

 let recentFolders = [];
 const monthOld = Math.floor((Date.now() - 31 * 24 * 60 * 60 * 1000) / 1000);

>>>>>>

This constant puts an upper limit on the age of a recent folder at 31 days.

So, if you want to have older folders in your recent list, you need to update this value, changing "31" to your preferred max age and then rebuild the omni.ja file.