Sorry... what stability problems. Can you be more specific on what you mean.

I have individual folders with around 100,000 messages in them, without issue

Stability problems: Means TB crashes!!! Some newsgroups have more than 10 times your 100,000, and like I have said before...The crashing is not because of add-ons because I hardly, if ever, use add-ons in TB. And yes, TB is current as always.

    It would be nice if TB gave one the option of downloading the oldest messages first because one would many times like to get the oldest messages before they are removed from the server...  They are usually the first ones to go.

Well if it crashes perhaps you could provide some crash ids. See https://support.mozilla.org/en-US/kb/mozilla-crash-reporter#w_viewing-crash-reports.

Only starting in safe mode from the help menu will exclude theme and add-on issues. It is no good of saying "I hardly, if ever, use add-ons in TB"

Note that Thunderbird is not a good choice as a replacement for a news server, and that unless you actively intervene in the process only message headers are downloaded (the same with all news readers in my experience.) upon selection of the message the actual message body is downloaded from the server.

Thunderbird also has a 4Gb limit per news group. Recent work has been done to remove the limit, but it may still be in the news part of Thunderbird as it gets almost zero programmer attention only experiment would tell you for sure. (NNTP is slowly dying if we like it or not).

I also note that most newsgroups with huge message counts are binaries groups and Thunderbird does a particularly poor job of reading yenc and recombining multipart messages. Perhaps a dedicated news reader is what you need.

I do supply a crash report every time TB crashes. About the add-ons: I should probably have said that I do not use them on TB. I had read somewhere about TB not being very good as a newsreader/binary group downloader, but...Let's just say I was stubbornly giving it a chance. I know that Usenet is a slowly dying breed, but as long as there's a gurgle of breath still left in it. I will be saddened to see it go!

    I am aware that TB only downloads the headers initially.  Thanks for the info about the 4GB figure.  That helps and it is something that I had not known before.  There is something about downloading a large # of headers from newsgroups that causes TB to constantly give one the "spinning torus" and (not responding) indicators that clear up for a second or two and keeps coming back.
    Anyway...  Well, I gave TB a good go at it; I guess it's time to move on to something better!  Thanks for your help and info!

most of the slow activity can usually be tracked back to anti virus interference. Excluding the Thunderbird profile folder from on access scanning is often a good decision.

I don't know what your crash ids are. without them I can not even look at "your" crashes.

I have 6000 or more photos emailed to me each day from IP cameras detecting motion an when I am close to the TBird limits I just get pages full of code that has not been translated into jpegs