Try these:

Rebuild the global database index (https://support.mozilla.org/en-US/kb/rebuilding-global-database).

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Clear the cache:

- press Ctrl-Shift-Delete (Mac: Cmd-Shift-Delete)

- set 'Time range...' to 'Everything'

- untick all items except 'Cache'

- clear > restart TB

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Disable the Accessibility Service:

- go to TB menu > Settings > General

- scroll all the way down and click the 'Config editor' button on the right

- click 'Accept the risk and continue' if that appears

- search for accessibility.force_disabled > change the value to 1

- click the check mark after making the change > restart TB

Note: this will disable accessibility features such as screen readers.

@Tim, Please post your results.

No improvement yet, but I have 2 questions. 1) Should the Enable Global Search and Indexer be checked or not when rebuilding the global database? 2) Does your suggested method of clearing the cache differ from simply doing it in preferences?

No improvement yet, but I have 2 questions. 1) Should the Enable Global Search and Indexer be checked or not when rebuilding the global database? 2) Does your suggested method of clearing the cache differ from simply doing it in preferences?

1. AFAIK, it doesn't matter.

2. If you do it in prefs, it may clear more than just the cache files but that wouldn't cause much if any harm.

> I was happily using TB 78.14.0 on Mac OS 10.9.5 and I switched to using TB 140.6

How did you update?

> Can something go wrong when you export the files from one TB into another?

How did you export/import?

I don't have any notes on how I updated and/or imported. The new TB is on a different mac. Most likely I downloaded TB onto the newer mac and then copied in the profile from the old TB, and updated TB afterwards.

Tim Hodgkinson said

No improvement yet, but I have 2 questions. 1) Should the Enable Global Search and Indexer be checked or not when rebuilding the global database?

1. AFAIK, it doesn't matter.

Enable Global Search and Indexer" must be enabled if you have deleted the global-messages-db.sqlite file in the profile folder and want to rebuild it.

I don't have any notes on how I updated and/or imported. The new TB is on a different mac. Most likely I downloaded TB onto the newer mac and then copied in the profile from the old TB, and updated TB afterwards.

So probably you transferred the TB 78.14.0 profile to the new Mac use it in TB 140.6? I advocate a test in a new Thunderbird profile: in your actual TB go to Help > Troubleshooting Information > Profiles > click on about:profiles... which will open a new tab "About Profiles". Click on "Create a New Profile", follow instructions in the profile wizard, then set up one of your email accounts and verify if Thunderbird is still running very slow

I carried out this procedure and TB seems to run normally. But of course it has no history, contacts, etc.

What was running slowly in your first installation? Fetching mail? Browsing folders? Searching? User interface elements? Something else?

A lot of things were running extremely slowly. Mainly at start-up it would say "No messages found" and in the bottom left corner "Loading message". This could go on for up to 18 minutes from start-up to viewing the inbox . Then each instruction like 'delete mail' or 'open mail' could take minutes

Do you have several accounts containing folders - Inboxes in particular - with thousands or ten thousands of messages? Launch the Activity Monitor before starting Thunderbird in your old profile and observe memory consumption (Thunderbird 146 on my iMac actually uses ~800 MB). Here is a checklist concerning memory usage problems https://wiki.mozilla.org/Thunderbird:Testing:Memory_Usage_Problems

The fact that your TB on the new Mac runs fluently in a new profile indicates that there is something foul in your old profile. Unfortunately the troubleshoot diagnostic test didn't help to find a culprit. Now it is very difficult to isolate the source of the slowness.

Certainly the best solution would be create all accounts in the new profile and transfer message files, address book files, calendar files, ...etc from the old to the new profile. If there are only IMAP, accounts message synchronisation would be automatic. POP accounts would need a bit more work. Local Folders can easily be transferred from one to another profile. But there would still be the question: are the amounts of messages too big in certain folders, in particular in Inbox and Sent folders? We don't have enough information to judge ...

Well I am trying to transfer the message files individually but I can't find a method that works. The address book file worked immediately. There is supposed to be an ImportExport Tools add-on which enables the transfer of individual folders but the download is corrupted every time. What is the simplest way of doing this? i.e. including the inbox. I tried manually placing folders into the appropriate place in the new profile but this has no effect.

Well I am trying to transfer the message files individually but I can't find a method that works. The address book file worked immediately. There is supposed to be an ImportExport Tools add-on which enables the transfer of individual folders but the download is corrupted every time. What is the simplest way of doing this? i.e. including the inbox. I tried manually placing folders into the appropriate place in the new profile but this has no effect.

The simplest way to transfer messages is to copy the mbox files into the local folders directory of your profile. Make sure that the mbox files have no extension. Re-start Thunderbird.

Using ImportExportToolsNG is another way to import messages. You shouldn't have to deal with a downloaded file. Add the add-on to Thunderbird by using the Tools > Add-ons and Themes menu option. Find the add-on in the add-on library and add it from within Thunderbird.

Well I am trying to transfer the message files individually but I can't find a method that works.

I didn't expect you to begin immediately to transfer message files without answering to my latest commentaries. My preferred method to transfer POP messages files to a new profil is manual, without using an add-on: • Copy the "Mail" folder from the old profile folder to the new profile folder. • in the new profile set up the first email address as POP account • go to Account settings > accountname > Server-Settings > Message storage > Local Directory and click on the "Browse" button (see image) • navigate to the "Mail" folder in the new profile, open it, select the corresponding "pop.xxx.xx" account folder (containing the old messages of the newly created POP account), click "Open" and restart TB. Thunderbird now should display all folders and messages for this email address. Proceed the same way for your other existing email addresses.

I can't say whether the import method with ImportExportTools NG is easier or faster than my manual method. For someone not familiar with the profile structures the add-on might help to avoid errors while transferring mailbox files from one to another profile.