https://support.mozilla.org/en-US/questions/1239651#answer-1170637

The release notes for TB 60.* describe the changes and fixes:

https://www.thunderbird.net/en-US/thunderbird/releases/

I am not looking for a list of "improvements and fixed issues". 60.3.0 will not support most addons and I can find no way of re-ordering folders.

The add-on mentioned in the first link of my first reply reorders accounts and folders.

https://support.mozilla.org/en-US/kb/installing-addon-thunderbird

You asked 'what this update was intended to do', and the release notes are the best place to start, as far as I know. The notes include this item: "IMPORTANT: Add-ons not marked as compatible with Thunderbird 60 by their authors will be disabled (this can be reverted via preference extensions.strictCompatibility)"

I was using the "Manually sort folders" addon by Jonathan Protzenko but since the update to 60.3.0 this no loger works. The screens appear blank. Firefox claims the fixes on GitHub are corrupt. Most of the addons available for Thunderbird will not work with the 60.3 update. It is difficult to see the loss of funcionality as an improvement. Without a solution, I and many others will be forced to find other software to manage and store e-mails.

I don't see any 'corrupt' signs on GitHub with Firefox. Maybe you just clicked on the link and it tried to install it in Firefox. Right-click on this link, Save Link As..., download the xpi file, then install it according to the instructions given above.

Thank you for correcting my error. Having now installed the file from GitHub, Manually sort folders is now working again. I still, however, wonder at the wisdom of releasing an update which renders so much functionality inoperative.

YourUpdate said

I still, however, wonder at the wisdom of releasing an update which renders so much functionality inoperative.

Mozilla is in the process of removing ALL of the support for Thunderbird add-on.s Their new model is web extensions and none of the old style extensions will make the transition without a full rewrite. The Thunderbird team are determined to hang out as long as possible before making any move to a new system and prefer never to rely on web extensions as they just do not do it for Thunderbird. As you see in V60 large parts of things that used to exist no longer do so changes were made to accommodate those changes. The next version will have even more dramatic changes to add-ons. The end story is unless the developer updates it it is not going to work in the future. So now is probably time to start looking at your add-ons and moving away from ones that are not actively developed.

I can assure you that not updating is not really an option either. With the was the encrypted web is going and the issues with new protocols being required for authentication with mail providers that it will not really be long before older versions just stop working with certain site and mail providers. Point in case is it appears AOL are moving to only accept oAuth authentication, along with yahoo. AOL is in TB 60 it is not in prior versions and never will be.

Thank you for the explaination. If Thunderbird must be updated I think Mozilla might do more to incorporate the functionality of popular addons into the main program.

YourUpdate said

Thank you for the explaination. If Thunderbird must be updated I think Mozilla might do more to incorporate the functionality of popular addons into the main program.

Two issues there;

1. Mozilla do not develop Thunderbird. It is entirely a community effort with a couple of contractors paid for out of donations to Thunderbird. So any suggestion of someone doing more obviously has a financial cost that really the Thunderbird community has to find.

2. Define popular. Seriously. The add-on site lists manually sort folders as 7th, but it also lists lookout as 8th and it has not apparently been supported on a version of Thunderbird since 12. Just because they are installed they are not necessarily enabled. I have 16 enabled add-ons. There are 43 disabled ones that count somewhere as installed. While my numbers are probably higher than most, I do not think the ratio of disabled to enabled is probably anything unusual.

I do appreciate the problems but believe that if Thunderbird hopes to remain in use, more must be done to keep existing functions working. My problems have just got a lot worse. The Lightning tab has dissapeared. Previous advice suggests re-installing this essential addon but the current version of Lightning says that it is not campatible with the current version of Thunderbird. I have the same problem on a Linux system with Thunderbird 60.2.1. I and no doubt others, urgently need a solution. Is this the same problem do should I open a new thread?

There are instructions for retrieving Lightning here:

https://support.mozilla.org/en-US/kb/calendar-updates-issues-thunderbird

If Lightning is present but hidden, typically on Windows, follow the steps in the section 'Lightning disappears after a Thunderbird update'.

Linux users with distro versions of TB, follow the steps in the section 'I am on Linux and Lightning disappeared'.

There are several posts on this forum about this issue.

Thank you again. I had seen earlier posts but was doubtful, first because they were some time ago and second because the available versions of Lightning specifically state that they are not compatible with current Thunderbird. I have temporarily resolved the problem by installing Thunderbird 52.2.1 and re-installing Lightning. Everything is now working. When I have ample spare time and subject to further advice, I will probably try updating Thunderbird but in the expectation that things will stop working again.