what type of mail-server are you using ? if its using last safe TLS, then TB v78 series will work by-default. TB v78 series will not work with those mail-servers which are still using older+unsafe TLS.

so you may have to lower this settings from 3 to 1 in TB v78 series, for TEST. but if TEST is not successful , change/revert it back to 3. "security.tls.version.min" setting, its inside "about:config" (aka: Config-Editor) ... it should be inside TB main-menu > Options/Preferences/Settings > General (in Windows & some Linux computers) or inside Advanced > General (in macOS computers).

make sure you added your mail-server's SSL cert into the TB's cert database (if it is a self-signed cert) , via using GUI of TB.

test your own mail-server from your Win10 computer, more info here, (also check PART-2 under the linked post).

you may use a second-TB for your own mail-server, use portable-TB v68.12.1 , see info here (see option-2).

I don't understand most of what you say, but I don't think the problem is my server. I have 3 computers, and the one in question (where I'm now writing) is a Dell laptop. The last update of T-bird was done automatically for me, in keeping with my settings, and that is exactly the time my inbox stopped receiving emails, so I think it must be something to do with the update, but I don't see a way to have it reversed or redone. (Btw, I can send emails from that/this computer.)

On my other two computers (Dell desktop and Lenovo laptop), I am able to run T-bird and retrieve emails, but I have other problems there. On one of them (desktop), my subfolders in my Inbox do not show up (in the other Lenovo computer, they do). Why is this?

Also, I exported and imported my address books from this (Dell) laptop to the other two computers. In both cases, the import did not recreate my groups -- how do I make that happen?

I apologize for these basic questions. I've been using T-bird for years and love it, but these glitches are somewhat problematic, as you can imagine.

Thanks for any help of these 3 issues.

let us assume these: Dell laptop = Laptop-1 Lenovo laptop = Laptop-2 Dell desktop = Desktop-1

ISSUE-1: your address-book was+is in Laptop-1, so, in Laptop-1 , you may have to downgrade the TB version first , fix address-book , then option-1: either update TB again to v78 series , or (option-2) keep using v68 series TB in laptop-1.

you MUST make backup copy of TB-"Profiles" folder, before you proceed.

v78 series TB does not uses older format of "abook.mab" file, this has your address-book , it converts into sqlite then uses sqlite file , and creates the "abook.mab.bak" backup file.

TB-downgrade options for Laptop1 are here (follow option-1 there). after downgrade, you have to do this, so that the "abook.mab" file which contains the address-book, becomes a simple address-book, also fix your groups so that its easy for the v78 to convert into "abook.sqlite" . v78 update/upgrade sofwtare not yet sufficiently intelligent , cannot anticipate difference types of changes that various users can do, and convert those changes correctly into sqlite format. so you have assist manually at this point . some things has to be copied manually. when you downgrade & have a working copy of TB, then you can manually tarnsfer/copy various settings/data.

ISSUE-2: in Desktop-1 , subfolder inside Inbox is not showing up : you have to re-SYNC it again : TB main menu > Tools > Account Settings > goto the mail-account which has that problem > in left-pane/side select "Synchronization & Storage" > goto "Message Synchronizing" section in right-side > select "Keep messages in all folders for this account in this computer" > click on [ Advanced ] button > select the folders that you want to sync > Ok.

PLEASE mention here, what exact version of TB is used by which computer. also mention are the TB update/upgraded automatically or you've manually update/upgraded.

also wait for other SUPPORTERs to help you further on this.

Issue 1: I am not presented with the option you said to use. Dell Desktop is using automatic update, now on 78.5.1

Sorry that ^ was meant to respond to Issue 2

my instructions/info were from my v68 TB. try to improvise & try to find similar word items & button in v78 series TB.

and please wait for some OTHER SUPPORTERS to help you on v78 series' exact button pressing sequences.